'YOU'RE PRIME candidates for joint custody," the lawyer said. "You both love your children, and you have been communicating about their welfare ever since your separation. That's all joint custody is, so why not write it into the divorce? A lot of people are doing it now, and it's really the best thing for the kids."

He was a kind, smart, reputable attorney. I trusted him, and he said it was the enlightened thing to do. Listen, I'm an enlightened person. Natural childbirth, assertiveness training, recycling; I keep up. Besides, I'd never been in a lawyer's office, much less a courtroom, in my life. He knew the game, and I didn't. Ninety days later, I raised my hand before a judge and swore the divorce settlement fair and equitable. Seven months later, I was in a hospital bed, under care for stress and depression.

A spate of rosy references to joint custody has been turned out in recent weeks by columnists who needed to get in some relevant copy the day after they'd seen "Kramer vs. Kramer," All this well-intended or unintended advertising for joint custody needs a health hazard label.

There isn't any happy ending for a story about splitting up a family when members of the family are cute little kids. "Kramer vs. Kramer" needed to stack the deck with a mommy who walks out for reasons which are perforce never sympathetically explored, doesn't say good-bye or bother to call for 18 months. If the audience had not been thus set up, no resolution to the plot could have been satisfactory enough to win all those pages in Time magazine.

But justice in real life is often a cloudier question. When you have a real-life mommy and daddy who both love their kids, the end of the story can be far more wrenching than the Kramers' conversation by the elevator.

I will not tell you any more than necessary about my grief or the gruesome details of the last year of my life; I am not the one who matters. It is for others, that I write; somebody needs to say this.

There is no such thing as joint custody. It is, at this point, a legal pipe dream. There is a substantial body of law and legal interpretation built up around the rights and responsibilities of parents who split into custodian and visitor. What happens in the wonderland of joint custody is nowhere defined in law.

So you take two people who have already established that they have sufficient antipathy toward each other on some level to shatter a family. Joint custody asks them to go on working together, even though their antagonism is proven, without benefit of legal guidelines.

Before I tell you this story, I must say that there are two sides to it, and I only know one of them. My former husband was a kind, loving father during our marriage. Some of what he became after the divorce I understand. But much of it I cannot. I will simply assert that he rapidly became a crummy father. He did not learn to make French toast. When the children opened his refrigerator and found only beer and sour cream, he handed them a five and sent them on a six-block walk to McDonald's. He moved them three times in the process of cohabiting with a series of ladies. Except for getting to know a lot of ladies, they didn't have much opportunity to make friends.

Then, as the odds are 10 to 1 that they will, the child support payments stopped. Not surprisingly, he was not eager to talk to me because the chances were high that I would mention the child support. His secretary and ladies were instructed to answer the telephone and tell me he was out.

These events are no uncommon after divorces, but I was caught in a special trap called joint custody. I did not have clear legal authority to make decisions about medical care, summer camp or any other aspect of the children's welfare.

The justification he offered for stopping child support was -- joint custody. He reasoned that, since he had the children two days a week, he had substantially the same expenses for housing them that I did.

He did run up quite a bill at McDonald's, but he was not heavy on buying them clothes or music lessons or bicycles. And he had been "out" when those decisions were made about medical care and summer camp, so he didn't feel obligated to pay any part of the bills.

So back I went to the kind, smart, reputable lawyer. He quite agreed that joint custody was not possible without communication, but the law, he assured me, has a remedy for this turn of events. He filed a petition for trial, and the news of victory came in the mail five weeks later, along with a bill for $370. The petition had been granted and a trial scheduled for the first available court date -- 11 months away.

The differences between mommy and daddy then were no longer just differences. They became Evidence. Mommy and daddy were v-e-r-y interested in what happened t the children when they were at each other's homes. Mommy and daddy, on their lawyers' instructions, kept little books in which they wrote down and dated bad things each other did.

My son started to relate a week-end adventure one day when his older sister sharply interrupted: "You're not supposed to tell her about that! Daddy said!" He began to cry: " forgot. He says not to tell to many things."

Mommies and daddies don't necessarily agree under any circumstances about how children should be raised. As long as they remain married, an incentive to maintain harmony motivates compromise or some other effort to give children consistent messages about how they should behave.

Under joint custody, no such motivation prevails, but the authority of both parents over the children's behavior does. What is likely to happen is that an innocent little kid is going to get very schizy trying to comply with contradictory messages.

Mommy believes people shouldn't own guns unless they need them for hunting, and my kids know I feel very strongly about it. But Daddy bought guns after the divorce, and they went out shooting bottles and crows on the weekends. Mommy has pretty strong convictions about teaching children to obey the law, and Daddy was letting a 15-year-old babysitter drive them around without a license. They needed to talk about the weekends. Had they been bad? Was Daddy bad? Was I mad at them? They were getting very confused.

Daddy met another lady then; I'll call her Lydia. Three weeks after he met her, he confided that he was in love and he was going to get married this time. The children were stunned. Lydia conducted strange religious services around a cassette recorder on the dining table and required the children to bow their heads and recite things they didn't believe. Who decides on religious training under joint custody? The law doesn't say.

Since Daddy had persuaded himself that he didn't owe any child support because he kept the children on weekends, he was in a bind because Lydia didn't like children. He solved it by continuing to pick them up on Saturdays and delivering them to a babysitter. The children said they liked the babysitter, but they began to cry on Saturdays. They said they were crying because it was time for Daddy to come, and they couldn't find their shoes. They said they were crying because they wanted to ride bikes, and their bikes wouldn't fit in Daddy's car. But I didn't believe them.

Then one Saturday, it was time for Daddy to come, and I couldn't find the children. They were hiding in a closet, crying. I began crying on Saturdays then, but the lawyer said it was only nine more months until the trial and I shouldn't jeopardize my case by denying his custodial rights.

But I did, the Saturday that Daddy didn't come at all. He telephoned instead and said he was out of town and a taxi driver would pick up the kids and take them to the babysitter. No, I said. He said what he did on his custodial time was none of my business. I said I didn't think that's what joint custody meant. But the law doesn't say.

He did indeed marry Lydia as soon as her divorce was final, and I was furious. And that's one of the tragedies. If it weren't for joint custody, his life would in fact be none of my business. But because he was making my children cry and because he still had rights and responsibilities for their care, it mattered a lot.

The children began to call me on weekends and ask me to come and get them. I sat alone in my house, looking at their toys and their cats, feeling their pain from across town, emotionally strangling on two words on a legal document.

Advocates of joint custody would, I imagine, contend that a lot of these events could have transpired under traditional custody arrangements. But when a parent faced with divorce is nudged by the legal system to assume joint custody, it's very hard to say no.

How can you face your kids or yourself if you say, no, I don't want custody? If joint custody hadn't been thrust at us, it's clear that I would have been custodian. Joint custody may be so important to some people that it's worth their taking the risk that they'll go through what I went through. But I haven't heard anybody advising them of the risks, and my middle goes tense when I read media reports promising joint custody as a happy ending to the story.

I made my own ending, as best I could. The children called me three times one day in July, asking what they could eat for lunch (Daddy and Lydia had gone out, and there was no food in the house), asking what they could do (Daddy and Lydia hadn't returned, and they were bored), asking me if I would come and get them (it was getting dark, and they were scared). I opened the sherry bottle after the second call. I paced the house and yelled at an imaginary Daddy. I started crying and couldn't stop. I called my doctor and asked him to admit me to a hospital for a few days. I was obsessed with anger, 35 pounds underweight, in debt from lack of child support and facing $5,000 more for trial costs, exhausted from inability to sleep. The doctors in the hospital gave me antidepressants and stern lectures about taking care of myself. Friends visited and helped me get up the nerve to do what I had to do.

This story took place on the West Coast. Three weeks after I got out of the hospital, I interviewed for a job at a much higher salary in Washington. and I got it. Two weeks after that, I loaded kids and cats on an airplane and moved 3,000 miles away from joint custody.

Was it kidnapping? Daddy isn't saying it is; he is apparently relieved to have us gone. The kind, smart, reputable attorney says he can build a case for relocation due to financial necessity. But the law doesn't say.