They're not yet telling patients to "take two aspirins and call Tel-Med in the morning," but dial-a-doctor is sweeping the nation.

The system of recorded medical messages, which began in 1971 as a San Bernardino, Calif., medical project, is about to have a slightly delayed celebration of its 200th installation around the country, even though the number may be closer to 250 by then.

In the general Washington area there are three organizations which make Tel-Med tapes available to the general public.

At Prince George's County Library, an estimated 10,000 callers a month plug in to the recorded 3- to 7-minute tapes.

Perhaps four times as many callers are dialing the tape number at Greater Southeast Community Hospital to hear messages on subjects ranging from what to do about baggy eyelids to helpful hints for a happier sex life. (There are separate tapes on male and female sexual response.)

There also is a Tel-Med library at Potomac Hospital in Woodbridge, Va.

Brochures are available which describe the taped subjects, indicating the hours calls may be made and the numbers to call. Each tape is assigned its own number so when calls are made to any of the Tel-Med branches, the same tape number will produce the same medical information. For example, a request for Number 302 will produce a discussion of "Effective Toothbrushing," whether in Washington, D.C., or Tulsa, Okla. -- so long as it is a Tel-Med center.

Kenneth Steels, a hospital administrator and self-styled audiophile, is the director of Tel-Med's home base in Colton, Calif.

The nonprofit program was started, he recalled, as a device to alleviate a chronic doctor shortage in the rapidly growing San Bernardino valley. By the time Tel-Med went on the air with 50 tapes in 1972, it "had become the talk of the California medical community" and its national acceptance was assured by the spring of 1973 when San Diego and Indianapolis, Ind., simultaneously instituted programs.

Now there are 310 tapes in the Tel-Med system and another 500 or so scripts from various member organizations which can be made available to other members requesting them.

Before a script is accepted formally and recorded at the California base, it must be screened, reworked, rescreened and approved by a medical panel, which must include at least one member who is a specialist in the field.

"Sometimes they go back and forth for a year before they're accepted," said Steele.

The idea, he said, was to anticipate the most common questions on a particular subject and give the patient a smattering of background to facilitate communication and treatment when he or she got to a doctor. The tapes are also useful for disseminating information to many people, rather than repeating it to patients one by one. Also, there are subjects that some people are too embarrassed to talk about, but -- given the anonymity of Tel-Med -- not too embarrassed to hear about.

The library is constantly expanding, Steele said, and scripts on alcohol and drug abuse, including the abuse of prescription drugs, are among 25 new tapes nearing completion. "We are also hoping to get into mental health and mental hygiene," he said, but it is "not a field of cut-and-dried medicine."

It can be difficult, he said, for example, to get panels of psychologists and psychiatrists to agree on something like a description or definition of schizophrenia.

The California operation serves now as a kind of clearinghouse, accepting scripts and suggestions from Tel-Med branches which they, in turn, get from callers or from physicians in their areas.

Tel-Med is nonprofit, but even the bare-bones cost of the program can be expensive, so not all of the Tel-Med branches have all the tapes. Some of Southeast's tapes, for example, are financed by the D.C. Society of Plastic and Reconstructive Surgery, as the recordings indicate. There also is the cost of operators when, as with Southeast, they are a specially trained team who do nothing but man the Tel-Med system.

The program at Southeast operates from 9 a.m. to 9 p.m., seven days a week. Other area programs are on more restricted schedules.

Fran Flemming, chief of the operators at Southeast, takes a no-nonsense view of the operation, admitting that a certain percentage of calls, especially for the tapes on sexual response, are of the "crank kids" variety. She says the operators are, however, adept at weeding out the younger children and, on the presumption that "we treat them as we'd want our children to be treated," either stall the titillation-seekers or flat out tell them certain tapes are for adults.

Some children, she said, will occasionally call asking the operator to "play that story for me," in which case they get a tape on children's good eating habits (Number 403) or the one on toothbrushing.

"It's better than a lot of things they could be into," says Flemming, and even when some of them come to snigger, they stay to listen. "It's a little like reading an encyclopedia."

An occasional tape will offer a flash of what might even be termed humor.

In the tape entitled, "Where Did I Come From Mama?" the narrarator tells the (old) story of the dedicated mother who, in response to that question, draws a long breath and gives junior the half-hour lecture. Whereupon junior says, "Yes, Mama, but what I wanted to know . . . well, Johnny says he comes from Chicago and I just wanted to know where I came from . . . "

Brochures are available by calling or writing The Greater Southeast Community Hospital, 1310 Southern Ave., SE, Wash. D.C. 20032, (561-9500), or from the Prince George's County Library system in Maryland, or Prince William County Library system in Virginia.