The Rev. Jesse Jackson yesterday urged about 500 teen-agers enrolled in the District's summer youth employment program to "say no to drugs."

"The number one threat to the development of this generation is drugs," Jackson told an antidrug rally of mostly 14- and 15-year-olds in the gymnasium of Winston Elementary School in Southeast.

Jackson was greeted with cheers and whistles when he was introduced by Mayor Marion Barry.

When Jackson asked those in the audience who had experimented with any type of drugs to come forward to the podium, about 25 teen-agers responded. When he asked how many in the group knew people who had died from or were in jail because of drugs, more than half the youths responded by standing.

"You are not going to become what you want to become if you are off in the twilight zone, if you are high," Jackson lectured the youngsters.

"The killers aren't wearing a sheet, the killers aren't carrying a rope, the killers are carrying a syringe, they're carrying PCP," he said later in his speech.

In March, during a similar speech by Jackson at Spingarn High School, nearly 100 of the 900 students in the audience stepped forward to admit they used or had experimented with illegal drugs.

In his address yesterday, the civil rights leader told the black audience to aspire to attend college or learn a trade and to become politically active.

"I am convinced that of all the forms of motivation there is nothing better than possibility as a motivator," Jackson told the youths, who are enrolled in an academic enrichment program that is part of the summer jobs program.

"Opening up the doors of opportunity is only half," said Jackson. "In addition to opportunity you need effort, and effort must be superior to opportunity."

Earlier in the day, 700 to 800 other participants in the city's summer jobs program attended a similar rally at the Fletcher Johnson Educational Center in Southeast, according to Matthew F. Shannon, director of the D.C. Department of Employment Services.

Two more antidrug rallies are scheduled today at Calvin Coolidge Senior High School and Armstrong Adult Education Center, both in Northwest.