Hilary Strong describes the annual three-week Edinburgh Festival (this year's is the 49th) as the ''largest arts festival on earth.'' Hilary Strong is the director, however, not of the festival itself, but of its ''Fringe.'' With its thousands of performers from 35 different countries, the Fringe is ''open access'' and ''unprogrammed.'' It is massive compared with the official festival, but it has never become the cuckoo in the nest, ousting the highly reputable incumbents. The official festival, which started just after World War II as an idealistic, morale-boosting attempt at international understanding through the arts, still attracts, from all over the world, top-ranking opera companies, orchestras, solo singers and instrumentalists, dance both modern and traditional, theater of the first order, and art exhibitions, which, if somewhat on the side, are always worthwhile. Among the 1995 main festival attractions are the Kirov Opera performing two Rimsky-Korsakov operas; the Abbey Theatre, Dublin, with ''Observe the Sons of Ulster Marching Towards the Somme'' - a play about Irish protestants by Frank McGuinness, a Catholic republican; the Berliner Ensemble performing Shakespeare's ''The Merchant of Venice'' in German; the St. Petersburg Philharmonic Orchestra, contributing two concerts to this year's celebration of Dvorak's work; and pianist Andras Schiff, performing his own mini-festival devoted to Schubert, with (in some recitals) the tenor Peter Schreier. There is a host of smaller classical recitals in addition to the large concerts. Dance includes Miami City Ballet's production of Balanchine's version of ''The Nutcracker,'' and Mark Morris - who seems to have now become a permanent festival fixture - with a program of new work and his ''wild and witty'' version of ''The Nutcracker,'' ''The Hard Nut.'' The dance program is, in fact, exclusively American, and its most daring aspect may well be Bill T. Jones's ''Still/Here.'' Jones's piece is on a harsh theme (how people cope with serious illness), and although its message is by all accounts life-affirming, the choreographer has nevertheless advised visitors (according to a newspaper interview): ''If you can't handle it, don't buy a ticket.'' Paradoxically, it seems altogether possible that this challenge will have people flocking to the Edinburgh Playhouse this weekend. They are certainly flocking paradoxically to see a Fringe play based on what is probably one of the more abject (but brilliantly written) ''cult'' novels of recent years, ''Trainspotting,'' by Irvine Welsh. It's a sellout. Performed with a mumbling dedication to its impenetrable street dialect by the reputable Citizens Theatre from Glasgow, the articulate wit of the novel is largely lost. All its repellent degeneracy is intact, however. Its subject is Edinburgh's youth drug culture. Bitter and self-abusive, this might be called ''Aversion Theater'' or the ''Theater of Bodily Functions.'' Either way, it makes the sugar-plum fairy seem like a sweet, childish fantasy - which, of course, it is. The Miami City Ballet's production of ''The Nutcracker'' was delicious, yet somehow short on fizz. The multitude of children (all Scottish) were enchantingly doll-like. By far the most memorable performance came when Myrna Kamara danced ''Coffee'' with a lithe, muscular, sensuous elasticity to out-Salome Salome. The Little Prince and Little Princess on their throne seemed unperturbed by this intrusion of originality on the scene. But then they were unperturbed period. And so they should be. ''The Nutcracker'' apart, I am struck this year by the thought that squeamishness is not a characteristic of those who attend this stupendous annual arts show. Many visitors seem ready - actually eager - to undergo some of the traumas, confrontations, and perturbations - as well as the entertainments and enchantments - that come under the banner of ''serious'' drama, dance, music, or art. Even the Fringe is by no means all alternative comedy, late-night cabaret, fireworks, and every conceivable form of ''street fun.'' Just before seeing ''Trainspotting,'' I had been to a truly remarkable one-man Fringe performance from South Africa. This was called ''Feedback,'' and its subject was food. Bodily functions were part of this, too, but the outrageousness this time was the fruit of gloriously energized hilarity and fantastical inventiveness. I was urged to go by a couple from New Jersey I sat opposite in a cafe, who praised it as ''indescribable.'' The ''one-man'' was Andrew Buckland. With his usual partner Lionel Newton unwell, Mr. Buckland did it alone, fabricating - by incredibly deft interweavings of movement, sound, speech, facial contortion - a wild (though morally serious) story of a rebellion of food and its needy supporters against waste, dearth, and a corrupt food industry. Everything (including the foodstuffs) was brought to life in a flood of astoundingly mirthful personifications. The flying cheeses, each with its own distinct, birdlike voice, were especially wonderful. It was spellbinding. So, too, was a six-woman play, all song and movement and rivetingly sincere acting, called ''Bondagers'' at the Traverse Theatre. Like ''Trainspotting,'' it is Scottish. First performed (in Glasgow) in 1990, its author, Sue Glover, has said it is about ''misuse of the land'' - a theme that seems modern enough. But these women belong to another age: The ''bondagers'' were female field-workers, victims of the centuries-long misuse of country people that is a still-resented historical memory in Scotland. In the end, it is not the survival of the land but the obstinate resilience of the women that stirs the imagination. They are hardly heroes, but they have indelible characters, hardy, funny, and stoical. Kathryn Howden's portrayal of the simpleton ''Tottie'' is an under-the-skin work of terrible sadness. As I write this, I am about to go to the first of the two Rimsky-Korsakov operas, ''The Legend of the Invisible City of Kitezh,'' sung in Russian. I am not sorry to see from the program that it has English subtitles. The footnote that announces ''Running time: approx. 4 hrs,'' does, however, suggest yet again that if there is one thing most needed by Edinburgh Festival enthusiasts, it is probably stamina. * Part II of Christopher Andreae's report on the three-week Edinburgh Festival will be published Aug. 30.