It's the last scene of the play, and a lead actor is alone on the stage comtemplating jumping off a bridge. But in the wings another actor is waiting, a bit player with a run-on role who will nonetheless, change the outcome of the play. Enter -- the dog, barking.

He -- or she -- chases the actor across the stage, grabs his pant leg and sends him halfway up a lampost. Exit the dog.

The play "Luv," by Mike Nichols and Elaine May, will open March 5 at the Rockville Civic Center, and this Saturday some 60 dogs -- from unknown puppies to dinner-theater regulars, Pekingese to Saint Bernards -- will audition for the role of "the dog." The chosen dog will get a contract for $100 to cover nine performances, plus rehersals. An understudy will also be picked.

"We don't care about the breed or the size," said Betty Clark of the Roundhouse Theater, which is staging the production and auditioning the dogs. "But if we find more than one dog that can do what we want we'll pick the one that looks best on stage."

Those dogs that don't get cold feet will audition for about six minutes each with actor David Di Giannantonio, who denies that he hates dogs.

"Usually I'm indifferent to them," said Di Giannantonio, "but when they grab the cuff of my pants . . . I just hope they have some tetnus shots ready. I've never worked with animals before, only vicious actresses."

The only dog that sent in a resume is Mr. Jagger, a Pekingese with a long list of dinner-theater credits, including the part of "the dog" in the Hayloft's production of "Luv," with Gloria De Haven. He has also played the role of a vicious dog in a training film for the U.S. Postal Service. Mr. Jaggar's ambition, according to his resume, is to play Sandy in a touring company of "Annie."

"But he usually plays sex farce -- in the nude," deadpanned Mr. Jagger's owner, actress Jelene Dudd.

Most of the hopefuls, however, are unknowns, family pets with little or no stage experience.

"Has Nathan been in a show? Well, at the Montgomery County Fairgrounds last year he won the prize for the largest dog," said 12-year-old Rachel McGuckin owner of a black Labrador retriever. Rachel expressed full confidence in Nathan's ability to play the role -- "He chases strangers all the time anyway," -- but denied that she had any Hollywood-style ambitions for him.

"I just like to take him places and have fun with him," said Rachel.

"My brother taught Chippy to bark and chase him when he's carrying a ball, said Suma. "So maybe if the actor in the play could carry a ball. . ."

The dogs who don't make it through the auditions shouldn't take it too hard. Pal, the famous Collie who played Lassie in the movies, was rejected at the first tryout because his coat was too scruffy. The next thing anybody knew he was a star, pulling down a thousand dollars a week -- in 1948 dollars.