Critics call them "Medical McDonald's," "Doc-in-the-Box" and "Kentucky Fried Medicine."

Supporters say that they give inexpensive, convenient and quick medical care, and they fill the gap between costly hospital emergency rooms and private physicians.

They are "emergi-centers," emergency clinics that in the past year have popped up in the Washington suburbs, mirroring a medical convenience center boom that started in the suburbs of California, Texas and Florida a few years ago.

Taking a cue from no-frills airlines and legal clinics, they are located in shopping malls or along major roadways. And, despite the dispute that the trend has triggered, there is one point of agreement.

"There's no question about it," says American Academy of Family Physicians spokesman William DeLay of the boom. "It does offer a serious competitive challenge to traditional medical practices."

There now are at least five of these emergency centers in Northern Virginia, six in suburban Maryland and none in the District, according to the National Association of Free-Standing Emergency Centers. In 1979 there were about 80 such centers in the nation; 750 are expected by the end of the year, according to the association.

At the Old Town Walk-In Medical Center on Washington Street in Alexandria, a pelvic examination is $3, a pregnancy test $3, and a tetanus shot $7. The charge for dressing a burn ranges from $4 to $15.

Similar treatment normally costs at least twice this much at private doctors' offices, and still more at emergency rooms, which must support the cost of expensive equipment for serious emergencies.

"It can cost $50 to walk through the door of the emergency room--just the initial registration," says Dr. Reinald Leidelmeyer, who opened his Fairfax Acute Care Clinic on Rte. 50 near Gallows Road.

Dr. Leidelmeyer had founded the emergency department at nearby Fairfax Hospital and had supervised it for 22 years before he opened the clinic.

Most of the centers are open an average of 12 to 16 hours a day, seven days a week, including holidays. There normally are five to 10 physicians who work at the clinics part-time, with usually one or two on duty at a time. They generally are supported by a nurse, a physician's assistant and a laboratory technician.

No appointments are needed, and most of the clinics will accept credit cards.

Most of the clinics will treat everything from sprains to flu to heart attacks, although with a heart attack case, the patient will be sent to a hospital as soon as his or her condition is stabilized.

The emergency clinics, which like to have patients in and out in about 45 minutes, cater to a transient and time-conscious society. Patients gave several reasons for prefering the clinics, ranging from no need for appointments, to lower prices, staying open longer, and the convenience of the clinics' locations.

Some patients say that there are no family doctors in their areas.

The success of the centers has raised concerns within the medical establishment. The centers' national association sets standards for its members, but some states seek to draw up formal licensing procedures that would include strict requirements and on-site inspections. Efforts to enact such regulations in Arizona, Florida and Indiana have been defeated.

Patrick Carroll, general counsel for the Massachusetts Hospital Association, notes that the registration fee in hospital emergency rooms is intended to spread the cost of necessary expensive equipment. He predicts that if the new centers substantially reduce the number of patients who use emergency rooms, the cost to the remaining hospital patients will soar.

But not all hospital officials expect problems to arise.

"They the free-standing clinics do not replace emergency rooms," says Peggy Pond, spokeswoman for the Fairfax Hospital Association. "We do not see that we are in competition with them. The proper use of an emergency room is for a real emergency. It's an expensive place to get a sore throat treated. We've always encouraged the use of private physicians for such things."

The centers normally skimp on costly, nonmedical extras. "You don't see two-inch-thick carpets here," says William Longstreth, acting administrator of the Old Town Walk-In Medical Center. "That just adds to the costs."

Most of the centers perform many of their own laboratory tests and develop their own X-rays, in order to save money and time. The Fairfax Acute Care Clinic, at 8301 Arlington Blvd., says it would cost $38, for example, to send out for a herpes virus culture test that it can perform for $5. The patient is charged $15.

The centers depend upon large volume to make their profits, and count on seeing about 60 to 80 patients a day, according to industry analysts. Few centers in this area have reached that goal. Typical is Emergency USA, which has been in business at Fair Oaks Mall near Fairfax City for the past seven months.

Emergency USA is owned and operated by 16 physicians, who for 10 years had worked at the emergency departments at three area hospitals. The first two months that their shopping-mall clinic was open, it treated an average of seven patients a day. By November, the number had risen to 20, and now between 30 and 40 patients are treated daily. The upswing in business was so encouraging that on March 1 the partners opened another center at the Wintergreen Plaza Shopping Center in Rockville.

Some physicians in private practice have extended their hours to meet the competition from the centers. Longstreth, however, contends that his center does not threaten these physicians because most of its patients do not have a family doctor and would go straight to an emergency room anyway.

Says Longstreth: "There is a need for all."