After Heavenly Cause won the LaTroienne Stakes by five lengths today, trainer Woody Stephens faced a great temptation.

Stephens had brought last year's 2-year-old filly champion to Churchill Downs with the idea of using today's race as a prep for the prestigious Kentucky Oaks next Friday. But now he is thinking hard about entering Heavenly Cause in the Derby and letting her try to emulate Genuine Risk's feat of last year.

Having trained horses for the last 41 years, Stephens is no dreamer. He was not seriously considering this course of action until he saw how Derby favorite Proud Appeal won the Blue Grass Stakes at Keeneland on Thursday.

"That was a bad race," he said flatly. "As bad as those bums looked, I don't think they'd scare anybody."

Heavenly Cause gave her trainer further encouragement today. She trailed her five rivals along the backstretch, circled them on the turn and drew away from runner-up Fiddleatune without any serious urging by jockey Pat Day. The victory boosted her career earnings beyond $450,000, more than any of the male candaidates for the Derby.

"The owners and I are going to talk it over a bit and see how she is tomorrow and Monday, but it looks like the Oaks to us," Stpehens said. However, he recognized that there might be one good tactical reason for choosing the Derby. In the Oaks, Heavenly Cause would have to face trainer Bud Delp's undefeated filly, Truly Bound, and might well lose. In the Derby, Stephens said, "It would be no disgrace to lose, but would be a feather in her cap to win."

Another Derby nominee gave the people around him some encouragement this afternoon. Mythical Ruler, who has spent most of his career racing in the bush leagues, captured the mile Stepping Stone Purse for his third straight victory of 1981 and earned a spot in the Derby lineup.

Mythical Ruler led all the way to score by four lengths over Classic Go Go, who won several stakes in Maryland this winter. The performance prompted 19-year-old jockey Kevin Wirth to exclaim, "I think he is a superhorse." Logic suggests that Wirth will be disabused of this notion next Saturday.

Proud Appeal's unsensational victory last week has prompted enough owners and trainers to try the Derby that Churchill Downs may have to invoke its rule that limits the number of starters to 20. Some 25 horses are currently listed as candidates for the classic.