''Oh, but that was long, long ago,'' said the Minneapolis-based composer, winner of the 1975 Pulitzer Prize for his song-cycle ''From the Diary of Virginia Woolf'' and now the contributor of the latest installment in the St. Louis Symphony's series of commissioned works.

''That's probably good,'' Argento, 58, said in a telephone interview. ''They say the instruments a composer really knows arethe ones he tends to be the most cautious about. I've been away from the clarinet so long, though, I've forgotten all my preconceptions. At least I think I have. I hope so. We'll see next weekend.''

Next weekend - specifically, Friday and Saturday evenings and Sunday afternoon - marks the world premiere presentations of Argento's ''Capriccio for Clarinet and Orchestra ('Rossini in Paris').'' The work was commissioned by the Symphony on behalf of its principal clarinetist, George Silfies, who will be th e soloist in the Powell Hall performances.

For all his disclaimers, one has to wonder how distant Argento really is from the clarinet, which he studied for one year in the mid 1950s while pursuing his doctoral degree in composition at Baltimore's Peabody Conservatory.

ALTHOUGH ARGENTO'S catalogue of compositions dates back to 1950, the new ''Capriccio'' is in fact the first work he's ever produced in which a solo instrument is featured. Granted, the stimulus for the piece came not from him but from Silfies, who was on the Peabody faculty when Argento was a student there and who has long been both a friend of Argento and an admirer of his music. But the composer says that over the years it's been his habit to turn down offers to write works for virtuoso instrumentalists.

''I've been asked to do violin concertos and piano concertos, but I never took the opportunity,'' he said. ''So why did I all of a sudden accept the challenge of writing a clarinet piece? Well, in the first place I figured it was time for a change of pace, because since 1977 I've been composing nothing but song cycles and operas. Secondly - and this is hard to explain, I guess - the idea of writing for the clarinet just, uh, appealed to me.''

Silfies says he remembers that Argento studied the clarinet at Peabody. But he never heard him play, and he doesn't recall hearing even a rumor about how well the composer did with his lessons. Still, Silfies knows that Argento retained at least a smattering of knowledge of the instrument's technical workings.

''ABOUT TWO years ago, several months after Dominick agreed to do the piece, I was beginning to feel anxious about it,'' Silfies said. ''So I wrote to him and asked him to send me a rough draft of the solo part. I said that if that wasn't possible perhaps I could see the first page, or the first line, or even the first note - anything, just so I could get started on it. So he sent me the first note scribbled on a piece of paper. It was a simple A above the staff, and alongside the note was a diagram showing me how to finger it!''

The tone of that correspondence is apparently reflected in the new ''Capriccio,'' but it's likely the piece would have been in the jocular vein even if Silfies and Argento hadn't known each other well enough to kid one another.

''When they told me I could have a work written especially for me I figured I might as well get something I really wanted to play,'' Silfies said. ''I knew exactly what I wanted: a nice piece that I could understand and that even my neighbors could enjoy, something with a strong tonal center and recognizable melodies, a listenable piece that wasn't flashy just for the sake of being flashy. And I wanted a piece with a title other than 'Concerto.' ''

SILFIES DREW up a list of several composers he thought capable of writing ''nice'' music for the clarinet. But his first choice was Argento, an admittedly conservative composer who's been committed to the tonal, lyrical idiom ever since he joined the faculty of the University of Minnesota in 1959. (''I wrote my share of belligerently dissonant pieces,'' Argento said when asked about his style. ''But that was when I was still in school, and I did it only to be fashionable.'') Silfies' motion that Argento get the commission was immediately seconded by the Symphony's music director Leonard Slatkin, who has known Argento for several years as a result of their collaborations at the Minnesota Orchestra's Sommerfest programs.

It was Argento who suggested the piece be a light-hearted one,''something that would be fun'' for the performers as well as the listeners.

''THE IDEA started with a trip to Italy in the summer of 1984,'' Argento said. ''I saw the revival of Rossini's long-lost comic opera 'A Voyage to Rheims,' and I was struck by how effective this music was. It was just an 'occasional' opera, intended only for entertainment. But it's so brilliant, especially in the ensembles. What appealed to me right away was Rossini's attitude about music, the attitude that says one can be a good composer even when you're writing something just for fun.

''You see, most of my music - at least, most of the music I'd composed in the 1970s and early '80s - is quite serious in expression. When my song-cycle 'The Andree Expedition' was premiered in Minneapolis in 1982 one of the local critics wrote that this is 'yet another installment in Mr. Argento's continuing preoccupation with gloom and darkness.' I'd never considered that. It's true that most of the texts I'd selected for operas or songs hadbeen about persons who led tragic lives. Of course, I just thought that these were the most interesting people to deal with. But then I wondered: Maybe I was always going back to the same cupboard for my ideas. And that's when I decided it was time to shift my vision.

''I'VE COMPOSED 13 operas, but the latest one - the 1984 'Casanova's Homecoming' - was my first comedy. Comic operas are rare these days, perhaps because we like to look at the darker side of life, perhaps because they're just so diffic ult to write. For one thing, they usually contain about a million more notes than the typical tragic opera - look at how thick the score of 'Meistersinger' is compared to the score of 'Siegfried,' for example. For another, the comic tone is very hard to strike - I mean, if you tell someone your mother-in-law was run over by a streetcar it's almost impossible not to get a sympathetic response, but if you tell a mother-in-law joke it's so easy to fall flat.

''Still, I need to try it. Since about 1982 I'vebeen purposely exploring the lighter side of human nature. 'Casanova's Homecoming' was one product of that. So was my 1983 song-cycle 'Casa Guidi,' a setting for mezzo-soprano and orchestra of texts drawn from the letters of Elizabeth Barrett Browning. And the 'Capriccio' fits into that pattern, too - it's just a good-natured piece, not something meant to change the world.''

THE ODD SUBTITLE of the ''Capriccio'' refers to the Rossini opera that in a way inspired it: ''A Voyage to Rheims,'' which will be given its American premiere this June by Opera Theatre of St. Louis, dates from 1825, and it was the first of several works the Italian composer wrote for the Paris audience.

The three movements of the ''Capriccio,'' which Argento says are structurally modeled on those in the clarinet concerto of Mozart, have labels related even more specifically to Rossini. ''Une Rejouissance,'' ''Une Caresse a ma femme'' and ''Un Petit Train de plaisir'' are all titles of piano pieces in Rossini's 1857-68 collection''Peches de vieillesse'' (''Sins of Old Age''). For Argento they suggest the bon vivant spirit that Rossini typically projected both with his music and with his life-style; they suggest the spirit to which the ''Capriccio'' aspires.

To be sure, artists in those good old days did experience unspeakable emotional agonies and horrible spiritual crises, albeit probably no more than did their non-artistic contemporaries. Artists, however, had a socially acceptable way to relieve their pain. At the very least, their work could be autobiographical in the sense that it represented their personal, troubled views of the world. But often it was openly cathartic.

The favorite character of the suffering artist was the suffering artist himself; he appeared in countless guises, and - human nature being what it is - he was usually represented as a sympathetic, even glamorous, hero. Artists still suffer. And perhaps because they genuinely need to, they still create works in which the very essence of artistic suffering is the focus of attention.

It is certainly the essence of Dominick Argento's ''The Voyage of Edgar Allan Poe,'' an opera from 1976 that has recently been revived by the Chicago Lyric Opera. The work was inspired by the mysterious journey that Poe took just before his death in October of 1849. Normally, the boat trip from Richmond, Va., to Baltimore lasted 48 hours; on this occasion, it took five days, and to date no one knows why. For that matter, no one knows for sure what it was that prompted the 40-year-old writer to throw himself into his last drug-and-booze binge. Almost immediately upon disembarking, Poe in effect vanished for another five days. He was found in a gutter, a mental and physical wreck. During the next four days, he was lucid only at the very end, when, just before he died, he quietly said, ''Lord, help my poor soul.''

But this is not at all a ''factual'' opera. The voyage it depicts is imaginary, one that takes place entirely within Poe's increasingly tormented mind. And the episodes that make up the legs of that nightmare voyage are drawn more from Poe's creative writings than from the documented events of his life. Argento and his librettist, Charles M. Nolte, have concocted a fascinating portrait with this work. Whether or not it is a portrait of Poe, however, is open to discussion. For all its many references to the familiar poems and stories, the opera seems an example of neither psycho-biography nor literary exegesis.Vis-a-vis Poe, it is a mere guess at what might have been going on inside the writer's mind at various stages of his career, and it may or may not be even remotely accurate. Vis-a-vis the ''suffering artist'' in general, especially the ''suffering artist'' as he has been transformed by neuroses endemic in the late-20th century, it is possibly very much on the mark.

The main event in this hallucinatory opera is a trial that happens during the second of the two acts. The defendant, of course, is Poe. The prosecuting attorney is the cryptic director of the theatrical company that shares space with Poe on the dream-ship; the judge is Rufus Griswold, Poe's back-stabbing ''friend'' and, after his death, literary executor; the witnesses are various persons, mostly women, from Poe's earlier years.

Poe is accused, as artists often are, of madness. ''Is it a crime,'' he asks, ''to go into the private world of visionary art?'' When asked if all artists, by definition, are mad, he replies: ''They are! And they choose the pain and shadows as others do the light. I choose the pain, the shadows. I do not see as other men. I cannot bring passions from a common spring! I look into the human soul, the human heart, and do not flinch to tell the dreadful truth I find.''

Later, Poe is accused as well of using his madness - his art - to the detriment of others. No, he did not kill his wife, he protests. But, ''yes,'' he says, ''I did feed on her vitality! She nourished my creations!'' ''Pity me,'' he says. ''I cannot think, I cannot love as others do. It is true. I murdered for my art. I killed all that I most loved, and sacrificed remembered bliss to feed my muse. It had to be. Have mercy on my life.''

A few moments later, as the scene builds to climax, Griswold-as-judge informs Poe that he is Poe's alter-ego, ''the guardian of your fame, your anima and nemesis, the eyes into your mind, your soul, your secret self.'' ''Now at last I know you,'' Poe says. To which Griswold replies: ''And thus you judge yourself, you judge yourself guilty.''

In the libretto for the original 1976 production by the Minnesota Opera, at this point Poe grabs a sword and savagely attacks the image of himself that he sees in a mirror. In Frank Galati's otherwise very cohesive production for the Lyric Opera, the macabre attorney stands ready, just before the blackout that leads to the final scene, to wield the fatal blow. It doesn't make much sense, considering the context. Poe is indeed guilty. But since the guilt - like the crime - exists only in his own mind, only he can mete out the punishment. Since he is both defendant and plaintiff, he must also be both judge and executioner. It is self-induced misery of this sort that defines the ''suffering artist.'' The frustration of being misunderstood and the indignity of not being financially compensated for one's efforts weigh but little compared to the burden of conscience that rests on the shoulders of the serious artist. To ''look into the human soul'' is, indeed, difficult. It is difficult to ''tell the . . . truth'' without flinching, even when that truth is not so terribly dreadful as some of those Poe encountered. And it is difficult to know what is the truth.

Argento and Nolte doubtless understand these unpleasant realities, probably all too well. Flowing as oddly yet as smoothly as a dream does, their opera is a wonder of craftsmanship; its chilling beauty owes equally to the words, which are often based directly on Poe's, and to the music, which adds up to one of the most lyric and impassioned 12-tone scores of the last quarter-century.

Galati's production, designed by John Conklin, is rich in chilling beauty, too, and the performance I heard on Tuesday - conducted by Christopher Keene, with tenor Donald Kaasch as Poe, baritone Richard Stilwell as his nemesis and soprano Ruth Ann Swenson as his muse - was perfectly stunning. More significant than the craftsmanship, however, is the work's statement.