"We were happy with our two children. We didn't see how we could improve on what we had," says Bill, who seven years ago at age 33 underwent a vasectomy.

"We both love children, and we're sorry to see ours grow up so fast, but we have no regrets. We didn't want any more children."

An estimated 500,000 American men a year are electing to be sterilized, a relatively simple, inexpensive 20- to 30-minute surgical procedure under local anesthesia, considered by advocates to be the most effective means of birth control -- except, of course, abstinence. The Association for Voluntary Sterilization quotes studies suggesting that as many as 96 percent of the men who have had a vasectomy would have one again.

A recent controversial study, however, has linked the operation to possible atherosclerosis, or hardening of the arteries.

Working with monkeys, two researchers -- pathologist Thomas Clarkson of the Bowman Gray School of Medicine in Winston-Salem, N.C., and physiologist Nancy Alexander of the Primate Research Center in Beaverton, Ore. -- have found that test animals given vasectomies developed higher degrees of cholesterol deposits in the arteries than those not sterilized. It's the body's reaction, they believe, to the blocked sperm that finds its way into the bloodstream.

A spokesman for Clarkson says that "while this is a good indication it would be true in humans, we have no studies to indicate that." But because the "results with monkeys look grim," Clarkson suggests men considering a vasectomy postpone it for a year until results are in from additional tests.

The National Institutes of Health has funded three studies now under way "on the possible risk of cardiovascular disease associated with previous vasectomy," says Ruth Crozier of NIH's Center for Population Research. The tests involve about 25,000 males.

"We consider the early monkey studies quite preliminary. But we feel that we have to answer the question of whether these monkey findings apply to men. So we initiated some human studies. It does not mean we think it applies to men."

"Men who opt for vasectomy -- cutting of the tubes that carry sperm to the penis -- do so for a variety of reasons, even though it probably means they'll never father another child, even if they decide they want to:

Like Bill and his wife, their families are as large as they want them.

They feel they can't afford any more children.

They're dissatisfied with other means of contraception, or don't want to be bothered with them for years ahead.

Their wives are having problems with the Pill or an IUD.

They've remarried after fathering one family and want an absolute limit on the number of children in the second.

Although men who have vasectomies, say experts in the field, usually are married and have at least one or two children, a bachelor occasionally will have it done. And couples who are childless by choice may decide to make their life style permanent. "If a couple elects not to have any children at all," says Mitchell Edson, chief of urology at Washington Hospital Center, "you have to honor their wishes."

Sometimes a man, particularly a younger one, is too eager for the operation and has to be talked out of it.

"I tell them, 'Your family life is all in front of you,'" says Gary Simpson, director of the Men's Center of Planned Parenthood of Metropolitan Washington. "One 18-year-old was dealing with so many women, and he didn't want any of them to say it was his child. That's the wrong reason to have a vasectomy."

The Men's Center doesn't actually perform vasectomies, but it does offer information and counseling and can refer men to clinics or doctors, usually urologists, for the procedure. Many phone in with what Simpson says are typical fears, even the most intelligent. I kind of bring them down.

"A guy's going to be uneasy about anything surgical in the testicle area. I tell them, 'No, it's not castration. It's very safe and easy.'"

The anxieties are there, agrees Mary Wright, director of counseling at Preterm, a "reproductive health" clinic that schedules vasectomies one night a month for $195. "Most men are not used to being poked and prodded the way women are."

An anesthetic, advises Preterm, "insures no pain, though the patient may feel a slight sensation of pressure." After the surgery, "discomfort is generally minor. Most patients will experience residual tenderness in the genital area for 7 to 10 days." Icebags and aspirin "are usually enough to provide relief."

Some men, says Simpson, worry that "once you have a vasectomy, you are no longer masculine or virile. That's the biggest myth." The surgery rarely interferes with sexual performance. Instead, says the Association for Voluntary Sterilization, it "usually results in relief and peace of mind for both partners. Many patients report increased enjoyment of sex when the fear of pregnancy has been removed."

Others, adds Simpson, think a vasectomy blocks off the flow of semen and prevents ejaculation. Also false. A vasectomy stops only the sperm, "and there's no change in the amount of semen ejaculated."

While those undergoing vasectomies cross all racial and economic lines, most men seeking information from the center, he says, "are rational, educated white males who have considered all options."

Few blacks show up, believes Simpson (who is black), either because of unawareness of the procedure or because of the fear of some blacks that sterilization is a form of genocide aimed at their race. "As a black, you always hear of genocide. This (vasectomy) is what people want you to do. Black people feel threatened about our children and our future."

Though the possibility of reversing a vasectomy appears to be growing, doctors and counselors continue to advise clients against the surgery if they have any doubts.

"If they can easily be dissuaded, that's not the person who should have it," says Washington urologist David Seitzman. "If I can talk them out of it, I'd just as soon do that. If anyone wanted the procedure done with the idea of it being temporary, I wouldn't do it."

While a vasectomy can be performed easily and quickly at a cost of about $200 to $400, the reversal is a much more intricate operation, may take three or four hours in a hospital operating room and can cost up to 10 times more. Health-insurance policies tend to cover the original surgery, but not the reversal.

Dr. Sherman J. Silber of St. Louis, the most prominent name in reversal surgery, says he has had a 90 percent success rate with 1,500 patients in the past seven years. But he doubts that most other practitioners have had the training to match his record. "That's the problem we face right now, the major reason not to think a vasectomy can be reversed."

Among Silber's clients who have changed their minds:

Couples who at 23 with two children "were trying to figure how to make it financially. They had no concept of the pay raises they would get at 33. Five or 10 years later, everything has changed, and they had always wanted more kids."

Parents who had "always only wanted two kids." Four months after the second child is born, "he has a crib death. When they change their minds right away, it's usually related to a crib death." For this reason, doctors and counselors often recommend you wait until your last child is at least six months old before you have a vasectomy.

Men who have remarried. "They almost always want a child in the second marriage." One 59-year-old client, says Silber, who had had a vasectomy 28 years earlier, underwent a successful reversal at the urging of his new 35-year-old wife.

Religious converts. New Mormons and born-again Christians "embrace the philosophy of bigger and bigger families."

Chances of complications in a vasectomy are considered slight and usually resolved easily with proper attention, though -- as the Association for Voluntary Sterilization points out -- "all surgery carries some risk." Among the possibilities, Preterm tells patients, are "infection," "excessive bleeding into scrotal sac," "excessive scar formation," "allergic reaction to local anesthetic" and "rejoining of tubes," a rare occurrence permitting resumption of the flow of sperm into the penis.

Washington psychiatrist Steven Lipsius, who offers counseling on sexual problems, thinks vasectomy practitioners should interview patients "intensely to make sure both spouses are in agreement" about the surgery. "No one should have a vasectomy under pressure from a spouse, or view it primarily as a sacrifice for the spouse." Otherwise, chances increase that "you're going to have trouble and feel bitter afterwards.

"If a man is very unsure of his masculinity -- and is asking questions really communicating that he views it as a castrating, emasculating kind of experience -- this has to be explored and the vasectomy deferred until it's resolved and he's more comfortable."

"To some men," he adds, "the feeling that they can impregnate a woman is important to them. It makes the sexual experience something not solely for sex." For them, if intercourse were only for sex, they might feel "guilt," or be "prone to depression not dissimilar to what some women experience in menopause."

A vasectomy, says Preterm, "should be elected only after careful deliberation and for sound, mature reasons after you have been fully informed of its nature, results and risks."

Concurs urologist Seitzman: "It's not a medical necessity. It's up to the individual. It really is a personal decision."