Nancy Reagan, "Hill Street Blues" actor Bruce Weitz and the producers of "Chemical People," the successful 1983 public television documentary about teen-age drug and alcohol abuse, disclosed plans for a sequel yesterday. Mrs. Reagan will introduce the film, as she did in the original.

"We've got to look deeper into the problems of drug and alcohol abuse and what it's doing to our precious children," the first lady told 120 White House luncheon guests, many of whom represented the original support groups of the 1983 film produced by WQED-TV of Pittsburgh.

Expressing the hope that the show will help young people who have become "slaves to drugs," Mrs. Reagan said "Chemical People II," to be broadcast on PBS May 7, will probe such drug-related teen-age problems as traffic fatalities, suicides, pregnancies, dropouts and felonies.

In his remarks, Lloyd Kaiser, president of WQED-TV, said, "We have to ask the question, 'Are all these symptoms of something that's deeper, is it perhaps loss of self-esteem, is it self-destructive values that young people have, attitudes we need to change?' "

After a lunch of chicken pot pie and orange ice cream, Kaiser told the East Room audience, "I think we will be more successful than the last time because we now have a five-station outreach alliance in place." After the earlier broadcast, he said, more than 8,000 communities around the country formed permanent task forces to deal with the teen-age substance abuse problem.

To be funded by the Corporation for Public Broadcasting, the hour-long film will be the first effort of the PBS Outreach Alliance of WQED, WETA/Washington, KCTS/Seattle and the Nebraska and Kentucky Networks.

PBS President and Chief Executive Officer Bruce Christensen, who sat with Mrs. Reagan, used the opportunity to acknowledge a $300 contribution President Reagan made recently after watching a PBS show about children with leprosy.

According to Elaine Crispen, Mrs. Reagan's press secretary, Mrs. Reagan told Christensen that Reagan wasn't sure how to make out the check or where to send it, so he called the station and carried on an anonymous conversation with the switchboard operator.

"She never knew it was the president calling," Crispen said the first lady told Christensen.

Weitz, who appeared in the original "Chemical People," said the 8,000 community task forces were tangible evidence that the 1983 film achieved results. He told reporters he was "a little skeptical" at first of Mrs. Reagan's drug-abuse crusade but "the enormous amount of time" she devotes to it "goes way beyond the call of duty . . . She puts in too much time for it to be lip service."

Weitz, who plays the grubby, growling cop Mick Belker on "Hill Street Blues," said he thought more people are on drugs in Hollywood because there's more money there. "My colleagues should be straightening out their acts the same as every business in the country," he said, adding that he wonders how many professionals, such as doctors and lawyers, are into drugs. "Nobody can tell me they aren't."

Weitz said Nancy Reagan "keeps telling me every time I see her that I have to get married. Just like my mother. I said, did she have anybody in mind, and she said, 'Oh, boy, could I fix you up!' "

Weitz, who growled into a reporter's tape recorder upon request and who gets married on tomorrow's episode of the series, said Mrs. Reagan was not talking about "Hill Street Blues."

"She's talking about life!" he said.