The first Breeders' Cup will not be run until next week, but in one respect it already is an unqualified success. It has attracted almost all of the top horses in the United States as well as many stars from Europe.

Certainly, the extravaganza at Hollywood Park Nov. 10 will constitute the greatest single day of racing in U.S. history.

For many weeks, there had been doubt whether the two best U.S. horses would take part in the Breeders' Cup. Sam Rubin, the owner of John Henry, was balking at the idea of paying the $400,000 fee to make his venerable campaigner a supplementary nominee. The owners of Slew o' Gold had reservations, too. But the lure of the biggest purses in American racing evidently proved irresistible to both camps. Rubin held a press conference in New York on Monday and announced, "I decided to do this because I feel I owed it to the horse."

John Henry will head a 13-horse field in the $2 million turf race, which looks as if it will be the strongest of the Breeders' Cup events. His rivals include last year's horse of the year, All Along, as well as the first three finishers in the Washington, D.C. International -- Seattle Song, Persian Tiara and Strawberry Road II.

Slew o' Gold has easier opposition in the $3 million Breeders' Cup Classic. His main foes in the eight-horse field are the California speedballs Precisionist and Desert Wine, and Preakness winner Gate Dancer.

Purses of $1 million are offered in five other Breeders' Cup events.

Two-year-old colts: Chief's Crown, the best of his age in New York, will be heavily favored in a field of 11, where his competition may come from the late-blooming California colt Image of Greatness.

Two-year-old fillies: Park Appeal, one of the best of her age and sex in Europe, will challenge a weak group of American fillies, headed by the California horse, Folk Art.

Sprint: Eillo, the New Jersey-based sprinter, faces a wide-open field of 13.

Distaff: Only seven fillies and mares were entered, but the quality of competition is extremely high. Princess Rooney faces Miss Oceana, Adored, Life's Magic and Lucky Lucky Lucky.

Mile turf: Lear Fan, one of the best milers in England, meets the formidable Californians Royal Heroine and Tsunami Slew.

Only a few healthy top-class horses are bypassing the Breeders' Cup, in almost all cases because they weren't nominated and the supplementary fees are discouragingly high. Neither Sabin nor Win will take part in the turf race; the fleet Debonair Junior won't run in the sprint.

But there were no major defections. "I think the Breeders' Cup is going to be what we envisioned," said its executive director, D.G. Van Clief.