Warning of "terrible new threats" posed by illegal drugs to the nation's schoolchildren this fall, Nancy Reagan yesterday called upon business leaders to join in a national campaign to solicit pledges from 10 million youngsters to "Just Say No" to drugs.

"I think the dangers to our children will move you," Mrs. Reagan told a special drug abuse prevention forum at the White House. "Your participation in this project can save many lives."

On cue as the TV cameras rolled, 11-year-old actress Drew Barrymore, in a tearful and at times inaudible recitation, apparently moved Mrs. Reagan sufficiently that she had to brush away a tear. Both the first lady and Donald Ian Macdonald, the president's special assistant for drug abuse policy, consoled Barrymore at one point by reaching up to pat her arm.

Barrymore, the youngest member of the acting dynasty that has entertained generations of movie and theatergoers, said that while her own offscreen life has been that of "just a normal person," she understands the pressures young people are under.

"One of the biggest pressures is the peer pressure to use drugs," sobbed Barrymore, the little girl with the big scream who found "E.T." in the closet in Steven Spielberg's 1982 hit. "Nobody wants to feel left out."

She said her family has made her "feel secure, but not everybody is as lucky. Some kids feel like they have nobody to care, and they feel all alone and scared. These are the kids who turn to drugs ... to feel better and get away from their problems."

Seeming overcome by now with emotion, Barrymore boohooed that in addition to two friends "I care about a whole lot," there are kids she does not know having drug-related problems whom she wants to help.

Speech over, she was all smiles as she signed an oversized pledge card.

Among her audience were Attorney General Edwin Meese III, Secretary of Education William Bennett, Assistant Secretary of State Ann Wrobleski and Drug Enforcement Agency Administrator John C. Lawn, all of whom also spoke.