BEFORE YOU give up on the human race, there is something you should know: Americans are lining up to adopt Down syndrome children.

Yes, according to the person who knows the most about it, 125 families are waiting to welcome children whose advent to birth-parents can be "shocking, overwhelming, depressing."

Janet Marchese, of White Plains, N.Y., runs the National Down Syndrome Exchange out of her house. She puts parents who don't want their Down syndrome children in touch with couples who do. The new parents are deeply instructed in what to expect.

Janet Marchese has placed 1,700 children in the last 15 years. She is one of those women who especially treasure children who aren't perfect. She has one herself.

The Marcheses are blue-collar and religious -- typical of the couples who take Down syndrome babies. Louis Marchese was a New York cop. Janet was a secretary who married at 18 and became the full-time mother of two. In 1976, a social agency that was arranging their adoption of two Korean children called and asked if they would take -- on a temporary, emergency basis -- a three-week-old Down syndome baby named TJ. They said yes and soon, in Janet's words, "became attached." They decided to adopt him.

Friends warned, "You will disasterize your family. You will ruin your kids' lives." The Marcheses joined a support group, learned about the baby's prospects, about infant stimulation. They learned about joy from TJ. He is now 15 and "a neat little guy" who reads and writes and plays baseball.

About the time of the adoption, a shift in attitudes towards Down syndrome children was setting in. People were no longer calling them "Mongoloid" for their slanting eyes. The agent of change was a landmark education law that required schools to give free education to all handicapped children. The Down syndrome children were, in short, "mainstreamed." The experiment was from the first a success, ending the isolation of children who can do much more than was thought possible at one time.

Janet Marchese began to get calls from people who wanted to know what to do about giving up their Down syndrome babies, born and unborn. Last year, she got calls from 63 women who were pregnant with Down Syndrome babies. Upon being assured that a hundred families were waiting to take them, all the women carried their babies to term and only two gave them up for adoption. Between 4,000 and 5,000 Down syndrome babies are born every year. Janet Marchese doesn't judge women who relinquish them. "They know what they can handle," she says.

The expense of her matchmaking became almost prohibitive. The monthly phone bill averaged $800. Janet Marchese went to work as a waitress and her husband moonlighted as a plumber. Many people gave her a pat on the back, but the Kennedy Foundation gave her money. First they paid the phone bill, then gave her $20,000 a year, soon matched by New York's JJJ Foundation.

She works closely with Dixie Lawrence, founder and Director of Adoption Options of LA Inc., who, like Janet Marchese, practices what she preaches. She has just adopted a year-old Down syndrome baby named Madison Eleanor. She places three or four Down syndrome babies a month. She makes sure the adoptive families haven't just despaired of finding a normal child and settled for what they can get. Families go to support groups, learn about Down syndrome at all ages. Couples who give birth to a Down syndrome baby can be frightened, she says, "Our families are emotionally and mentally prepared."

Dixie Lawrence is delighted with Madison, even looks forward to her teens. "I don't want a normal child. I had two and they drove me crazy. I won't ever have to drive to Alabama to get her out of jail. She'll hang out and be sweet."

There is, finally, Colleen Roth of Northwood, Ohio, who with her husband Dudley adopted Karen -- who is 19 now and has the development of an 18-month old child. She is in diapers, can't walk very well, or talk. Colleen Roth is delighted with her. She replaces the Roths' birth-child, who was born with cerebral palsy and other severe ailments and died at age 7. Karen is blind. So is her adoptive mother. Dudley Roth is also legally blind.

"People do not understand the pleasure that goes with this child," says Colleen Roth. "She has the most precious giggle. You know you are making her day."

Colleen Roth won a 1990 Jefferson Award for her multiple volunteer activities for the blind and the handicapped. "Don't write about me in a way that would make people feel sorry for me," she says. That may be possible, but how can the rest of us keep from feeling small?

Mary McGrory is a Washington Post columnist.