Q. Our son, unusually strong when he was born, was an independent infant who didn't like to be cuddled or restrained, and now he is a stubborn 3-year-old, feisty and aggressive with us and our live-in nanny.

Although he is articulate, cheerful, well-behaved and obedient at school, and behaves like any other active 3-year-old when we visit friends or go to a religious service, he's a different boy at home. He thrashes with such strength it takes two people to give him medicine or change his clothes or diapers, and he hits, throws and bites when he is bored or frustrated.

My husband, the live-in nanny and I try to keep him occupied at all times, but sometimes laundry must be folded, and dinners must be cooked. This is when he flushes something valuable down the toilet or throws the phone receiver at his 6-year-old sister.

Even the pediatrician dreads our visits. Our son is totally uncooperative during his exams, which are frequent since he is prone to ear infections.

We discipline him by telling him that he has done wrong and send him to the time-out chair, but many times he simply won't stay there for more than a minute before he is back in the room with a grin and an apology, saying that he will never do it again. And within 10 minutes he will hit his sister and the pattern starts anew, or she retreats, for fear of being hurt.

We know it's normal for a 3-year-old boy to be active and to be curious about the way the world works and we don't even mind his "experiments" -- such as flushing things down the toilet. But we don't want him to throw, hit, or bite nor do we want our family characterized by the "good" child and the "bad" child.

If a child misbehaves all the time, you first look for a physical problem that might cause these emotional symptoms, but if he just misbehaves in certain situations, you have to wonder what's inviting this aggression. In this case, you're almost surely seeing a little fellow who is desperate for tighter limits.

Children feel very uncomfortable -- even unsafe -- if their boundaries are too distant or inconsistent. This makes them push and push to find out where they stand. If you don't draw firm lines now, you and your husband will find yourselves turning into rigid, authoritarian parents when your son is a teen-ager, simply because you won't be able to stand it anymore.

This works the other way too. Parents who start out as autocrats usually become terribly permissive when their pliant little children turn into teen-agers who can't stand it anymore. These are the ones who rebel with a "Yeah, and who's going to make me?"

It's much wiser for parents to walk down the middle of the road. You'll get clipped a little in both directions, but it won't be dangerous, and it will give you a much happier household.

A number of changes need to be made, and all of them depend on your giving your son honest-to-goodness respect. He deserves the chance to learn how to play on his own, without constant attention to keep busy. That only tells him that he's not good enough, or smart enough, to entertain himself.

He also deserves the right to be held accountable, to learn that responsibility and freedom have equal weight. At this point he only knows that he can do what he wants, because you don't really mean what you say. It's time to realize that the grown-ups are in charge. You're bigger and older and smarter than he is. You pay the mortgage. You make the rules. Your son has no right to hit his sister, his parents or his nanny, with anything, ever; no right to bite anybody; no right to throw anything at anybody, or to flush things down the toilet, valuable or not. Every child tries this toilet stunt sooner or later, but when he does it to get attention, it's a hostile act, not an experiment, and he knows that too.

If you, your husband and the nanny respond swiftly and fiercely to even the slightest misbehavior, your son will come around in two to three miserable weeks, although you can expect him to be even more aggressive at first, and to have occasional relapses later.

From now on you have to react to any aggression with a "Don't You DARE Do That!" and then march him straight to the corner, clasp your hands firmly on his elbows -- tight to his sides so he can't hit you -- and back away as much as you can so he can't hit you when he kicks. And you don't say one word, no matter how wild he gets. He -- and you -- stay in this corner for five to 10 minutes, or as long as it takes for him to calm down.

After that, you walk him to his room, and tell him that you'll call him when he can come out. Even three to five minutes will be enough to make him realize that you are in charge, but he's sure to challenge you at first. When he comes out too soon, you have to march him right back in, without comment, and if he screams, kicks or bites, take him straight to the corner and go through this routine all over again.

The minute he waits until he's called, however, welcome back your good little boy, and make a fuss over him, as if he never did anything wrong -- and never will. Your son, like all children, loves attention, and if he gets more of it for being good than for being bad, he'll be good much more often. Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.