Q. "Would you please write about the growing sexual discovery in the under 3 or 4 set?" asks a Bethesda mother.

"What are the acceptable parental responses? This is one of those uncomfortable areas for me, and guidance would be appreciated."

A. You don't say what's going on, but at 3 and 4 (and 5), it's usually as much as a child can get away with.

The joy of sex is imprinted in everyone as surely as the DNA. It's nature's way to keep the species going and a pleasant way it is.

It is however, a pleasure that begins in puberty. That would be as unlikely as a child who would only coo and gurgle for his 14 years, and then start talking in complicated sentences.

Sex, then, takes practice. From birth -- probably before -- a child is as curious about his body as he about everything else. The most perfunctory diaper change gives an infant some slight exhilaration. Naturally.

Within a few months a baby -- boy or girl -- will touch the genitals and learn to linger there when there's nothing else to do, although enough adults will look askance to make him more discreet in a year or so. The fondling still will be casual and not too frequent, however, both because diapers are so bulky and because a child has so much else to explore.

By late 2, children begin to notice each other, as well as themselves. You can expect a little girl to be not only envious, but outraged when the newly-trained boy next door unzips his fly and dares to urinate as far across the sidewalk as he can. The humilation is even greater if she has never seen a boy with his pants down and wonders why she hasn't grown a penis yet -- or even worse, if she lost it.

Because we only can draw conclusions from the knowledge we have, it's important for your child to know as much about sex as he knows about, say, nutrition.

That's why you let a young child see other members of the family naked and arrange for him to watch a few babies -- boys and girls -- when they are bathed.

With candor, with simple, direct answers to his questions and some marvelling on your part over the wonderful sexual differences in animals (inclluding people) a child can understand sex better and a daughter gets over any jealousy she may have. In fact, she is so proud of her body you may find bright blue ballpoint circles on the bottoms of all her dollies in a couple of years. (She will swear, of course, that she has no idea how those vaginas got there.)

This use of clinical terms is another way to be straightforward about sex, but expect any names to sound obscene when they come from the mouths of babes. No matter what words you child uses, he still sounds like a barker on Fourteenth Street, because in the nursery school years a child thinks anything between the navel and the knees is naughty and that's just what he likes. By 4, the very word "toilet" will make him fall into a heap of giggles.

This joint interest in sex and in elimination actually begins at 3, perhaps because toilet training has had a child's underpants going up and down like a yo-yo. You may think this has no connection with sex, but at this age he confuses the two, although you've told him they're not the same.

The matter of undressing before friends -- under any pretext at all -- also starts at 3. Even though this is natural to a child, it isn't condoned by a parent and you have to keep a sharp eye out for the closed door. If he shuts it when he plays with friends, you can bet he's playing a game he shouldn't. When you knock on the door and walk in, the game is likely to be show and touch.

Of course you're going to feel uncomfortable, but it only calls for a "Come on, gang, get dressed. Underpants stay up and dresses stay down." If you pretended you didn't mind, your child would be confused by the tension he would see in your body -- one of those famous "mixed messages" psychiatrists talk about.

The same applies, of course, to mastrubation, but usually it's private enough for you to ignore. Certainly every child mastrubates occasionally, but if it becomes the solace whenever he feels rejected or lonely or bored or under stress, then he needs a lot more attention, more activities and perhaps some psychotherapy.

This is not a problem for most children, however. By 5 they get more prudish and dignify their sex games with a story line like "doctor" or "hospital" -- a prime chance to examine bottoms. Now they take rectal temperatures with everything from a popsicle stick to a soup spoon. If you forbid the practice firmly -- and this is what you should do -- the game loses its charm quite quickly.

In the first grade (occasionally before), sex begins to be sublimated, and reasonably pure romance takes over.

We know one precocious 5 who was enraptured to meet our visitor -- a handsome, blue-eyed fellow of 15.

Weeks later and miles away she sent him this postcard:

"I am your same age and size too .

I wish you were here .

I love you. From Prue ."

The straightforward approach dies hard, but at least it's less direct than it was at 3 and4.