Pensy the cat has urinary trouble; Heidi the dog has parasites. A mixed Lab named Tiger is nursing a broken jaw, and a cat in shock called Becket has felt the business end of a fan belt.

Saturday night at Rockville's Metropolitan Emergency Animal Clinic is not for delicate sensibilities. Lying prostrate in newspaper-lined cages, variously sedated or shocked or depressed, are a half-dozen animals.

There's Sam the cat, awaiting surgery, having possibly eaten carpet, and a nameless German shepherd who's been smashed by a speeding car. They make no noise as humans scurry to help them.

Technician Colleen Davies, in a faded green smock, holds a dehydrated Himalayan cat in place on a stainless steel examining table while her colleague Charles Hatcherson draws blood with a hypodermic. The sample suggests that the cat has suffered acute kidney failure. "Prognosis poor," read scribbles on the animal's chart.

Dr. Steven Schwartz, a bearded and by this time bedraggled veterinarian who has been on duty since noon --and earlier did time at another clinic--takes a break in a closet-size office. It's 9 p.m. and in a few minutes he will start exploratory surgery on Sam, carefully inspecting the animal's intestines for any sign of foreign materials.

But now Schwartz puffs and puffs on a stubby cigarette. "Call for you," says the receptionist, Lee Putnam, peeking in. Schwartz pushes a flashing button and picks up the phone.

"I'm afraid your cat has been severely damaged," he tells Becket's owner who has just phoned for the news. "His chances of making it are very slim. Unfortunately, I recommend that you put him to sleep." Schwartz goes over the grim formalities. Later, Hatcherson will perform the procedure with a lethal injection of tranquilizers.

By 11, Dr. James Hurley, Schwartz's replacement, has arrived and Schwartz has briefed him on the night's activities.

"I'm going to go home and eat dinner," Schwartz says, doffing his coat and stethoscope. "What will I have? It depends on whether I stop at Roy Rogers, 7-Eleven or McDonalds's" He trudges into the night.