Yet the audience that attended the premiere in London's Queen Elizabeth Hall inApril of that year was hardly prepared for the psychological impact ''Eight Songs for a Mad King'' would have on them. As will doubtless be discovered by persons who witness the Davies-supervised performance of the work Tuesday evening in Washington University's Edison Theatre, 16 years later the piece is still very capable of giving listeners a jolt.

The music's excursions into ear-jarring dissonance came as no surprise to that first London audience. Deliberately harsh and ''ugly'' sounds were par for the course in British avant-garde music during the turbulent '60s, and by the end of the decade Davies had already demonstrated numerous times that he was willing to spice his generally tonal and tuneful work with clashes of the most garish sort.

Nor were the work's appropriations of music of the past - most notably early-18th-century songs and 1920s-style swing - in any way unexpected. Trained in historical musicology as well as composition, Davies had worked with paraphrases of older music since his undergraduate days at the Royal College of Music in Manchester. Among his earliest published pieces are the 1957 ''Alma Redemptoris Mater'' for wind sextet, which is based on a sample from the Sarum plainchant, and the 1958 ''St. Michael Sonata'' for largewind ensemble, which incorporates into its texture fragments of the traditional Gregorian Requiem setting. His early references to medieval music were more subtle than obvious, but by 1964 Davies was unabashedly dealing with full-blown and only thinly-disguised musical quotations, using them as sonic dramatis personae whose ''personalities'' often changed drastically during the unfolding of a composition.

What was disturbing about ''Eight Songs for a Mad King'' was its brutal depiction of one of the most feared of human conditions: insanity.

The protagonist is the British monarch whose political ineptitude helped spark the American Revolution, who in 1788 suffered a major mental breakdown (now thought to have been caused by an inherited metabolic disorder) and who steadily lost touch with reality until his death in 1820. Davies' ''Eight Songs'' represent him near the end of his long life, in the last nine years of which the madness was so out of control that his son had to assume all royal duties. King George apparently was in possession of a small mechanical organ, and lore has it that in his final years he spent a good deal of time trying to teach his pet birds to sing the melodies encoded into the perforations of the instrument's metal discs.

In 1966 Randolph Stow, the author of the poems that make up the libretto for ''Eight Songs,'' actually heard a demonstration of the mechanical organ. ''It left a peculiar and disturbing impression,'' Stow writes in his program notes for the piece. ''One imagined the King, in his purple flannel dressing-gown and ermine night-cap, struggling to teach birds to make the music which he could so rarely torture out of his flute and harpsichord. And trying to sing with them, in that ravaged voice made almost inhuman by day-long soliloquies. . . ''

Stow's unsettling impressions were made graphic in the music Davies composed in February and March of 1969.

The king - portrayed by a baritone - not only sings in more or less normal fashion but also croaks, whines, bellows, cackles and occasionally screams at the top of his lungs. He's accompanied by an ensemble made up flute, clarinet, violin, cello, percussion and keyboards. ''Eight Songs'' is a theatrical piece: King George is costumed in full regalia, and four of the instrumentalists are cast as ''birds'' who spend the entire performance in cages. There are, of course, interactions between the king and his avian serenaders - some of them are poignant, some pathetic and some violent. The stage action does not pretend to realism; rather, the song cycle is a fanciful essay on madness that explores the issue from the points of view of both the insane person and his disconcerted observers. The ''beauties'' of the piece have to do not at all with the sounds but with the potency of the multi-level message and the craftsmanship with which Davies' music is constructed. On the surface, at least, ''Eight Songs for a Mad King'' is nothing if not shocking.

''But I wasn't out to shock anyone,'' the soft-spoken 51-year-old composer said in a telephone interview last month.

''It turned out that way, of course, but that was not my intention. By their nature, I think, many persons are shocked by art that deals with unpleasant subject matter. I suppose that's good, because it helps them get to know themselves better. Certainly back in 1969 insanity was not something that was generally discussed in polite society. That's still the case today, both in England and in your country. And so 'Eight Songs for a Mad King' continues to shock people.''

The best book on Davies and his music was written in 1982 by Paul Griffiths, a critic on the staff of the Times of London, for Robson Books' ''The Contemporary Composers'' series. In the interview chapter, Griffiths notes that there are two ways in which music can be shocking, one resulting from the sheer originality of the work and the other from the grotesqueness of the subject matter. Regarding the latter, Griffiths suggests that one reason why many of Davies' compositions from the late 1960s and early 1970s were considered shocking was because they went ''ever deeper into the human psyche.''

I asked Davies if he is as concerned now with exploring profound psychological states as he seemed to be a decade and a half ago. He said that he was, but he added that there are major differences between ''Eight Songs'' and his current projects.

''Yes, the psyche is still important to me,'' he said. ''But it changes over time, or at least our perceptions of it do. Sometimes we don'trealize how much it changes - in our own views - until long after a certain 'plumbing of the depths' has been done.

''My third symphony was premiered in February of this year. It's a purely abstract work, although its structure - like the structure of late-18th-century music - is in essence theatrical, with one theme dramatically pitted against another. While this piece is hardly similar in content to the 'Eight Songs for a Mad King,' I think that it actually probes more deeply into the workings of human nature.

''When I wrote the 'Eight Songs' I wanted to explore the idea of madness in order to expand my expressive possibilities. That was the beginning of it for me. By the time I finished 'Miss Donnithorne's Maggot,' a theater piece [1974 that deals with madness in a different way, I felt that I'd accomplished what I'd needed to. My expressive range had been expanded considerably, and as a result I found that my music no longer needed to be so hectic and frenzied. I discovered, too, that my technique had become more controlled, and more pointed. The means were a bit surer - I could hit the target more easily, with a better aim.

''My own life has changed very much since 'Eight Songs.' It was shortly after its premiere, you know, that I bought my house on Orkney.'' He went to that remote island off the Scottish coast for ''a real silence, not just the synthetic silence I found in the city. Moving to Orkney was a necessary liberation for me. It wouldn't work for everyone, of course - it depends on how well you can stand your own company. But I know that it affected my music in a very significant way. My writing grew more inward, more reflective. My music still has its dark side, but now there's also a bright side to it, and in a piece like the newsymphony these two aspects are intermixed. Human existence similarly has both a dark side and a bright side, and I would hope my recent music reflects that duality.''

The brighter side of Davies' musical personality will be evident in the other work that will presented on Tuesday evening. It's his 1978 ''Le Jongleur de Notre Dame,'' a ''masque'' in one act based on a 13th-century French poem about a humble street entertainer who finds that all he can offer as an homage to the Blessed Virgin is his ability as a juggler. The non-singing title role will be played by British juggler Johnny James, and the band of children that accompanies his entrances and exits has been recruited from the Young People's Orchestra sponsored by the St. Louis Conservatory and Schools for the Arts.