In 49 other states, the Kentucky Derby might be considered America's greatest horse race. But here, the Derby and the other classics are thought to be of trifling importance when compared with the Santa Anita Handicap.

Except that nobody calls it that. The race is known simply as the Big 'Cap, and its allure is so strong that 70,000 people are expected to jam Santa Anita Sunday to see its 46th running.

In almost every other state, the great showcase attraction is a 3-year-old race: the Florida Derby, the Preakness, the Belmont Stakes. A handicap race for older horses grew into the West's biggest event partly out of necessity. Santa Anita has never been able to attract the large numbers of good 3-year-olds who traditionally prepare in Florida and New York for the Triple Crown events. But it was able to get older horses to come here with an elemental lure: money.

The Big 'Cap was offering $100,000 purses even in the 1930s and '40s, and great horses like Seabiscuit and Citation ran in it. It is the oldest continuously run $100,000 race in the country, only now it is a $500,000 race. That purse makes it a natural objective for the best older horses, and its last four runnings have been won by champions--Affirmed, Spectacular Bid and John Henry (twice).

It is doubtful that any horse of that quality is entered in Sunday's 19-horse field. But in view of the weakness of the handicap horses in the East, the winner may lay a justifiable claim to being the best racehorse in America.

Bates Motel, a colt named after the site of the carnage in the movie "Psycho," may earn that distinction. A big, late-blooming 4-year-old, he had never won a stakes race until two weeks ago. But in the $200,000 San Antonio Handicap, he swooped around his rivals with the kind of acceleration that evoked memories of Spectacular Bid and covered the 1 1/8 miles in a swift 1:47 flat. Trainer John Gosden's chief worry is the weather: Bates Motel can't run in the mud, and Santa Anita has been seeing a lot of rain lately.

Besides the weather, Bates Motel has to battle tradition. Jockey Bill Shoemaker has won the Big 'Cap 10 times; trainer Charles Whittingham has won it five. A European import named The Wonder could make their totals 11 and six, respectively. The Wonder compiled a distinguished record in France last year, and won a prep race here last week with consummate ease. He probably will be the post time favorite Sunday.

Other contenders in the Big 'Cap lineup include Swing Till Dawn, Caterman, Exclusive One, Wavering Monarch and It's the One.