In row after row of padlocked drawers in an unmarked laboratory in McLean sit thousands of vials of green, yellow and purple tablets, LSD seized by Drug Enforcement Administration agents from Long Island to Los Angeles.

The pills are not remnants of the 1960s, when "acid" was often the hallucinogen of choice among college-age, artistic-minded and middle-class whites.

The LSD in the DEA's lab has been seized in the last four years, evidence that the drug is making a strong comeback on college campuses and among working- and middle-class whites in Boston, Philadelphia, New York, Miami, Los Angeles and San Francisco.

"There is a whole new generation of kids growing up today who refuse to believe the horror stories the generation before them suffered with LSD," said Dr. Edward Franzosa, chief chemist at the DEA.

They're finding out in growing numbers what the drug is like, authorities say. Emergency rooms in hospitals in 26 metropolitan areas monitored by the Drug Abuse Warning Network of the National Institute on Drug Abuse have admitted 4,378 people suffering from overdoses of LSD in the last four years. Three deaths from LSD overdoses have been reported in the past three years.

While those numbers do not compare with the thousands of deaths and mental hospitalizations attributed to LSD in the 1960s, the DEA estimates that there are five to 10 times as many LSD overdoses now than during most of the 1970s, when the drug was out of fashion.

One reason the number of LSD-related deaths remains low is that the doses of the drug sold on the street are much weaker than those available in the 1960s, when the average dose contained 180 micrograms. Nowadays, dosages run between 10 and 30 micrograms.

The strongest dose sold on the streets is a purple tablet containing 40 micrograms of lysergic acid diethylamide.

"These low dosages are still not safe," said Eric Rosenquist, chief of the Dangerous Drugs Unit in the DEA's Office of Intelligence. "But they're probably why we're not seeing the incidents of really bizarre behavior we saw from use of the drug in the 1960s, when people thought they could fly and jumped out of windows."

Rosenquist said that there are about 1.2 million Americans who now use hallucinogenic drugs, including mescaline, peyote and LSD. He said his agency estimates that LSD traffic now runs about $20 million a year, nowhere near the sales of drugs like cocaine, marijuana, heroin, PCP and Quaaludes, but a number that is sharply rising.

Not only has the dosage of LSD changed, so has the method of its use. In the 1960s, people generally took LSD when they were alone or with one other person to experience "the trip," where colors swirl, the senses blend, music acquires an aroma and colors carry a sound.

In the 1960s, users took LSD as often as three or four times a week, which made the practice more dangerous because the drug can linger in the blood as long as a week and in fatty tissue even longer.

Today, LSD is generally regarded as a party drug by users who often take it once a week in combination with cocaine or alcohol.

The main reason that the DEA is alarmed over a comeback of LSD is that users develop a tolerance for the drug and may begin turning to higher doses, Rosenquist said. "The temptation is there right now among LSD users to increase their inputs. The time to really start worrying about LSD is when street doses start rising to tablets containing 100 to 150 micrograms."