Some of the newest things in birth control are some of the oldest.

More and more, trials, tests and studies are tending to limit use of available oral contraceptives to fewer and fewer women.

(Most recently, studies indicate that women on the Pill are more susceptible to a buildup in their systems of the class of drugs known as minor tranquilizers--the diazepams--which include Valium and Librium and their ilk. Such a build-up can lead to dependence, even addiction, without a woman doing anything more than following her doctor's instructions. This build-up can and does occur without the Pill's help, but the new study shows that elimination of tranquilizers takes twice as long for women on the Pill.)

But the Pill has also been linked in studies to cardiovascular problems and several forms of cancer. And the IUD's relationship to problems like pelvic inflammatory disease or tubal pregnancies has compromised both its credibility and its popularity.

It has seemed to many women that the medical community has lagged in its recognition of the shortcomings of the Pill and the IUD and has dragged its feet on research into other easier, better methods.

Now, however, many physicians and researchers are rekindling their old enthusiasm for things like diaphragms, condoms and spermicidal foams and jellies.

"If you look at contraception through the ages," says Dr. Elizabeth Connell, health-care specialist in "contraception-sterilization-abortion" for some 20 years, "you'll see that the barriers are among the oldest we've had."

They included, she says, "all sorts of fascinating things--fumigating kettles, pessaries made of such things as elephant or crocodile dung mixed with honey." Also condoms made from sheep intestines and an array of mechanical devices that, in effect, were the earliest chastity belts.

Connell concedes that she was typical of her colleagues in her loss of interest in fitting diaphragms "after the Pill and IUDs came along, because we thought we had better methods. I think a lot of us just sort of downgraded (barrier methods) but given today's medical climate, there is brand-new interest both here and abroad because the barrier methods are totally safe and highly effective--which many don't realize."

"Some recent studies," she says, "have shown that if you use a female method (a diaphragm and gel, for example) and a male method like a condom, you have a rate of efficacy equal to the IUD and, in some studies, actually approaching that of the Pill. And it is a totally safe method."

There is also laboratory and some clinical evidence that the combination helps prevent sexually-transmitted diseases like herpes and perhaps even gonorrhea and syphilis. The spermicide nonoxynol-9, the active ingredient in most creams, jellies, suppositories and foams, appears to provide the anti-STD action.

Connell founded family-planning clinics in New York's Spanish Harlem in the '60s and now teaches gynecology and obstetrics at Emory University School of Medicine, Atlanta, Ga. She does not recommend a single contraceptive method for all women.

"You owe it to any woman," she says, "to go through the entire contraceptive cafeteria, as we call it, talking about benefits and risks. The most important thing is for a woman to look at herself in terms of her age, medical problems she may have, whether or not she is a smoker. Also in terms of her ability to handle certain methods.

"If a woman has a drug or alcohol problem or a psychiatric disease, then she's much better off with a method that does not require input from her. You have to have an insight into what she can handle, what her partner can accept."

Connell, who is writing a United Nations position paper on birth control, tends to disapprove of the cervical cap, a favorite among women's groups. She believes that the three-month-lasting injection of the controversial drug depo-provera will turn out to be safe, useful and, despite some of its side effects, popular among large numbers of women.

The cervical cap, she believes, is not useful in wide application because of its difficulty in fitting, potential for damaging the cervix and now, because of the threat of toxic-shock syndrome, apparently dangerous in its most popular feature: the fact that it can remain in place for days, even weeks at a time.

(Even the diaphragm, left in too long, is being linked to toxic-shock incidence.)

What about a contraceptive for men?

"People think," Connell says, "that no work is being done, that it's a male-chauvinist plot. There is a lot of research, but it's very hard to turn off male fertility. And the side effects to date have been absolutely overwhelming: feminization, destruction of libido, inability to have an erection or ejaculate."

Besides that, researchers have discovered an "escape phenomenon": A sperm-supressant suddenly stops working. Without a daily sperm count, "You don't know about it."

The good news, says Connell, is that research on a line of disposable, spermicidal pessary or sponge-like female barriers is well underway. Some may be available within a matter of months.

Barriers like the cervical cap, she says, are "a nice idea for a small highly motivated group, but I really wonder in the long run if it will have any major advantage over the more traditional diaphragm. It won't change the world picture in birth control."

The new barriers are something else again. "I agree," she says, "that if there is a way to do it without drugs, that would be best . . . So I think if we had an esthetic, safe and effective barrier, not specifically related to the time of intercourse quite as directly as suppositories or condoms, I think it would have a world-wide impact."