Until the past month, Noble Dancer II was one of the most brilliant, consistent grass runners in America. But he comes into Saturday's Washington, D.C. International with two conservative last-place finishes, performances so dismal that something drastic must be bothering the horse.

The cause of his problem is becoming increasingly apparent. Noble Dancer is in love.

At the age of 6, when most high-class horses are enjoying a life of sensuality on a stud farm, Noble Dancer is manifesting an unseemly interest in the opposite sex. "He's more masculine and more aware of fillies than just about any horse I've ever had," trainer Tommy Kelly said. "He nickers and he hollers whenever he sees a filly."

Noble Dancer's roving eye never affected his performance until last month. Since he came to the United States in 1976, as Norways representative in the International, he has rarely had to compete against members of the opposite sex. Most American trainers have a phobia about entering fillies against males.

But Angel Penna harbors no such sexist prejudices. Penna trained from 1971 to 1977 in France, where fillies routinely run against colts, because there are few big-money races limited to females. After his filly, Waya, trounced the best members of her own sex this summer, Penna did not hesitate to enter her against Nobel Dancer and other males in the Man O'War Stakes at Belmont Park.

On paper, Noble Dancer was clearly superior; he had, after all, run within a neck of the great Exceller, the best grass runner in America. But on the track, Nobel Dancer might have been overly interested in Waya's charms, while Waya kept her mind on her business. "I don't think she notices whether she's running against colts or fillies," Penna said. Waya beat Noble Dancer by 15 lengths.

Two weeks later, in Belmont's Turf Classic, Noble Dancer was doubly distracted, by Waya as well as the invading European filly, Trillion. Waya defeated Trillion in a photo finish, with Noble Dancer trailing them both.

"I don't think he's enough of a male chauvinist," Kelly lamented.

Waya's victories were demonstrations not only of her superiority over Noble Dancer, but of the European treatment of fillies over the American.

In Europe, where Waya launched her racing career under Penna's tutelage, fillies are toughened by regular competition against males. And they can compete successfully at the sport's upper echelon. Fillies have won the Prix de I'Arc de Triomphe, the continent's most prestigious race, three times in the last seven years. European fillies have captured the International twice in the last five years, and either Waya or Trillion may make the record three-for-six on Saturday.

By contrast, American fillies almost never beat males in championship races. The reason must be the fact that trainers have view them as members of the weaker sex, who must be pampered and shielded from the toughest competition. The performance of the businesslike Waya against lovesick Noble Dancer hardly leads credence to this notion.