Doctors are testing a powerful new male contraceptive that suppresses sperm production so powerfully it causes complications in some men, but still could become the first successful male birth control pill.

But the substance in its first human trials on eight volunteers at Vanderbilt University also caused impotence in five of the men, and four reported "hot flashes" like those that affect many women.

The Vanderbilt study is nonetheless "an exciting beginning," and the compound shows more promise than any previously tested as a potential male contraceptive, Dr. William Crowley Jr. of Massachusetts General Hospital in Boston said in an interview yesterday.

It is possible, he said, that all the apparent problems will be overcome so quickly that the compound, called LHRH-A, could be ready for Food and Drug Administration approval in five years.

At the least, he says in an editorial in today's New England Journal of Medicine, the substance seemed to prove "safe and effective" and the undesirable effects disappeared when the drug was stopped. This is in contrast to the "bankruptcy" of other approaches to male contraception.

The Nashville, Tenn., experiment was headed by Dr. David Rabin and is reported by Dr. Randy Linde, Rabin and five coworkers in the New England journal.

The substance is a synthetic developed at the Salk Institute in California, an analog or chemical cousin of a hormone released by the pituitary gland at the base of the brain. That hormone, variously known as either GnRH (for gonadotropin-releasing hormone) or LHRH, powerfully stimulates the sex glands, the male testes and the female ovaries.

But it also, perversely, has been shown to inhibit sperm production in some circumstances in animal tests.

It was on this basis that the Vanderbilt group began work with the Salk Institute synthetic, 200 times more powerful than the natural hormone.

To do so, they recruited men who had decided to have vasectomies, but agreed to postpone them to take part in the study.

The eight male volunteers were taught to give themselves injections of the contraceptive hormone, though it also could be given as a nasal spray or nose drops.

The same or similar substances are in fact being tried in nasal sprays in tests as a female contraceptive by suppressing ovulation in the United States and Europe. Early reports, especially from Sweden, are optimistic, Rabin said yesterday. "So there is the possibility of what you might call a unisex contraceptive."

The Nashville male volunteers gave themselves LHRH-A for six to 10 weeks. In all, sperm production fell precipitously. In six, it almost disappeared.

Production of testosterone or male sex hormone also fell, causing the impotence and lack of sex drive in some men. Treatment was stopped after six or seven weeks in five men because of these effects.

Trials are already under way, however, to give another group of men both LHRH-A and small doses of replacement testosterone to keep sex drive and potency normal. If these tests work, and if other effects can be controlled, Crowley said, future development could come quickly. "Within the next year or two, we'll know whether this next step works."