Artificial-heart recipient William J. Schroeder took a lap around the Churchill Downs race track today, jostling along in a horse-drawn surrey with the wives and widows of other men whose hearts were replaced by mechanical pumps.

In what has got to be the strangest public event in the history of the controversial heart experiments, Schroeder, the women and surgeon Allan M. Lansing of Humana Hospital Audubon led the hospital's fourth annual "Cardiac Walk," in which 200 "graduates" of Audubon's cardiac unit strode the dirt path of the Kentucky Derby.

Schroeder, a stroke victim who must use a wheelchair, wore an "I Walking" T-shirt. His wife, Margaret, clad in a jogging suit, sat behind him and draped a hospital blanket over him in the early morning chill.

The shoulder bag-sized pump that powers Schroeder's heart with puffs of air lay on the floor beside him. "We had to put winter-grade oil in it this morning," Lansing joked to reporters and photographers.

Schroeder, whose second stroke left him in May unable to talk for a month, responded to reporters' questions with a clear "feeling great." Riding along with the Schroeders, below the twin spires that have come to symbolize the lure of winning on a long shot, were the other women whose husbands have gambled on Dr. William DeVries' artificial-heart experiments:

*LaVonne (Jinks) Burcham of LeRoy, Ill., whose husband, Jack, died of internal bleeding 10 days after receiving the plastic and aluminum device in April.

*Juanita Haydon of Louisville, wife of Murray P. Haydon, who has spent more than half of a year in intensive care, intermittently dependent on a respirator, plagued by bleeding problems, a transient stroke and pneumonia.

*Una Loy Clark, widow of the first artificial-heart patient, Seattle dentist Barney Clark, who died in 1983 after seizures, bleeding and mental depression during 112 days with a Jarvik-7 heart.

Yesterday in an "emotionally overwhelming experience," Una Clark met Haydon and Schroeder for the first time.

She said she felt a rush of memories "good and sad," because Haydon reminded her of her late husband, who was never well enough after receiving the implant to leave the hospital.

She visited Schroeder at his apartment across the street from the hospital. She said she kissed his forehead and told him, "I just want you to know how much I love and appreciate you and what you're doing."

Schroeder looked into her eyes and replied, "We do it for others."

Clark said she was startled by improvements in the artificial-heart machinery. "When you would go up to my husband and touch his shoulder, you could feel the heart. And I mean it was strong. Any place on his body, you could feel that heart pounding. And the noise from that drive unit!

"When I took Mr. Haydon's hand, I couldn't feel a thing. I was so shocked." Such improvements, she said, are why the program should continue, despite a growing chorus of medical experts calling for the experiments to be slowed or stopped because of the high incidence of strokes.

". . . I don't think we are going to find out anything more about it by quitting.