Joyce Nalepka could hardly believe her eyes. There she was, in the midst of 17,000 youngsters in Washington's Capital Center arena, at a rock concert - and 80 percent of them were drinking alcohol, smoking marijuana joints, or taking cocaine.
The staff of the arena were simply watching it all happen. Outside, parents sat in hundreds of cars waiting to take their children home.
In fact, Mrs. Nalepka had not intended going to the concert at all. Her former baby sitter, a boy she had stopped employing when he started taking drugs , had called her begging for a ride because his parents were busy. He had offered extra tickets.
She liked the boy, and finally decided to take him. She took along her own two children as well.
''It woke me up,'' she recalls. ''So I started to work, drafting legislation to close down shops selling drug paraphernalia and telling other mothers about the drug problem.''
That was in 1977. Now Mrs. Nalepka has been joined by many other parents. She is senior vice-president of the National Federation of Parents for Drug-free Youth (NFP) and is enthusiastically supported by the Reagan White House - and by Nancy Reagan in particular.
The NFP has about 4,000 affiliated parents groups around the country, and a mailing list of 40,000 people. It works with such other parent groups as PRIDE in Atlanta, forming a nucleus of parents against drug abuse that has attracted attention in Western Europe and at the United Nations as well.
''We have to take back control over our children's lives,'' said Mrs. Nalepka at the NFP headquarters after the PBS-TV program on fighting drug abuse entitled ''The Chemical People.''
''We have to get on the phone, know where our kids are going, who their friends are, who their friends' parents are, who is supervising their parties. We want to encourage alcohol-free and tobacco-free parties.''
Thirty-six states now ban paraphernalia shops, and the NFP hopes that all 50 will before long.
Helping Mrs. Nalepka is tall, dark Carolyn Burns, who has had two boys in treatment for drug abuse.
When one son was in the Straight center in St. Petersburg, Fla., she and her husband made 18 round trips from Maryland in 16 months to attend parent conferences and to see him. The family spent about $30,000 on drug treatment for both boys.
Eventually so many children from the Washington area were in Florida that Mrs. Burns and other parents organized a branch of Straight in northern Virginia for 130 of them and raised half a million dollars to launch it.
Some professional health educators criticize the parents movement. They call it simplistic and say it tends to see progress where little of lasting value has been achieved.
''Not everyone is in favor of parents taking direct action,'' says Bernie McColgan of the National Institute on Drug Abuse. ''A common professional view is that you mustn't exert too much discipline in case you alienate the children.''
''Well, what do the critics want?'' Mrs. Nalepka asks. ''For us to keep on waiting? It's too late to wait.
''Look around you, at movies and television and billboards, all pushing alcohol and glamorizing tobacco and drugs. . . .''
The NFP ''hot line'' number is 800-554-KIDS.
The address: 1820 Franwall Ave., Silver Spring, MD 20902, USA.