When the Breeders' Cup was run for the first time last fall, nobody knew quite what to expect of it. The idea of a one-day extravaganza of championship races had as many detractors as it had boosters.

But that day at Hollywood Park electrified the racing world. It produced so many brilliant performances, so much drama and controversy that it surpassed even the most optimistic expectations of its creators. As a result, there are few skeptics left to doubt that the second Breeders' Cup, at Aqueduct Saturday, will be memorable.

The seven races will offer $10 million in purses, decide several Eclipse Awards and probably decide the horse of the year title, and include some of the truest international championship events that have been run. The participants include horses from the world's greatest racing stables (the Aga Khan, Robert Sangster and Sheik Mohammed al Maktoum among them) and almost all of the world's most famous jockeys (including Steve Cauthen, Lester Piggott and Yves Saint-Martin).

The world's richest race, the Breeders' Cup Classic, will bring another rematch of the four horses who have battled each other inconclusively throughout New York's major fall stakes races: Chief's Crown, Vanlandingham, Track Barron and Gate Dancer. "It's as good a race as you'll ever get," said Track Barron's trainer, LeRoy Jolley. "It deserves to be $3 million."

If Vanlandingham and Track Barron hurt each other in a battle for the lead -- which usually happens when they meet -- their rivals will have a chance for redemption. Chief's Crown, who had been discredited after losing all three of the Triple Crown races as the favorite, probably could earn the horse of the year title with a victory Saturday. Gate Dancer could atone for blowing the Classic last year, when he looked like a winner turning for home but lugged in so badly through the stretch that he wound up getting disqualified from second place.

As competitive a race as the Classic is, the best horses at Aqueduct Saturday may be in the grass races. A European journalist here said flatly, "This is the best and deepest group of horses that we have ever sent to your country. Your American horses have no chance whatsoever." The quality of the foreign entries proves that the Breeders' Cup has won international acceptance.

It is quite possible that the brightest star to emerge from this day of racing will be the English filly Pebbles. In her last start she faced the English Derby winner Slip Anchor and blew him away with a powerful move in the stretch. Her performance was acclaimed widely as the best of the year in Europe. Trainer Clive Brittain said, "She could be the filly of the century -- or even the horse of the century."

If Pebbles makes a successful transition to racing on Aqueduct's tight turf course, she figures to dominate the 1 1/2-mile event; if not, the European colt Shernazar can win it.

The foreign forces are even stronger in the Breeders' Cup Mile. Rousillon is regarded as the best 4-year-old miler on the continent and Shadeed as the best 3-year-old miler. Rousillon may be the better of the two, but he won't be helped by his stretch-running style and his No. 13 post position. Shadeed has more speed and may have a tactical edge as a result.

The other events on the Breeders' Cup program are likely to be dominated by trainer Wayne Lukas, which is only fitting. The Californian has dominated U.S. racing this year, winning a record 61 stakes from coast to coast.

Lukas has his strongest hand in the 1 1/4-mile Distaff, where Ladies' Secret will be seeking her ninth straight victory and will have the help of two stablemates, Life's Magic and Alabama Nana. The trainer said he waited until the last minute before deciding against running Lady's Secret against males in the Classic in a bid for the horse of the year title.

"In a year that's been up and down, she's the only one who's shown any consistency," Lukas said. "We think she's the best horse in training today."

Lukas has another three-horse entry in the race for 2-year-old fillies, consisting of Twilight Ridge, Arewehavingfunyet and Family Style. Any of them is capable of winning, and the entry will be an odds-on favorite against an otherwise weak field.

Lukas has a strong two-horse entry in the Breeders' Cup Sprint: Mt. Livermore and Pancho Villa. They may be favored but they hardly have a lock on the race. The Sprint is probably the most competitive event of the day. One of the most formidable entrants is Precisionist, a brilliant colt from the West Coast who is coming back from an injury and hasn't raced for nearly five months.

Lukas also has two colts in the Juvenile -- Ketoh and Louisiana Slew -- but this appears to be the one race in which he is overmatched. Mogambo is likely to be a strong favorite on the strength of his 9 3/4-length victory in the Champagne Stakes at Belmont Park two weeks ago. CAPTION: Picture, Chief's Crown will be running in today's Breeders' Cup Classic at Aqueduct, part of a seven-race series that will offer $10 million in purses.