"We definitely didn't know 5 or 10 years ago that alcohol affected the teen brain differently," said Dr. White, who has also been involved in research at Duke on alcohol in adolescent rats. "Now there's a sense of urgency. It's the same place we were in when everyone realized what a bad thing it was for pregnant women to drink alcohol."

One of two brain areas known to be affected is the hippocampus, a structure crucial for learning and memory. In 1995, Dr. White and other researchers placed delicate sensors inside living brain slices from the hippocampi of adolescent rats and discovered that alcohol drastically suppressed the activity of specific chemical receptors in the region.

Normally, these receptors are activated by the neurotransmitter glutamate and allow calcium to enter neurons, setting off a cascade of changes that strengthen synapses, by helping to create repeated connections between cells, aiding in the efficient formation of new memories.

But at the equivalent of one or two alcoholic drinks, the receptors' activity slowed, and at higher doses, they shut down almost entirely. The researchers, led by Scott Swartzwelder, a neuropsychologist at Duke and at the Veterans Affairs Medical Center in Durham, N.C., found that the suppressive effect was significantly stronger in adolescent rat brain cells than in the brain cells of adult rats.

As might be predicted, the cellular shutdown affected the ability of the younger rats to learn and remember. In other experiments, the team found that adolescent rats under the influence of alcohol had far more trouble than did tipsy adult rats when required repeatedly to locate a platform submerged in a tub of cloudy water and swim to it.

Dr. Swartzwelder said it was likely that in human teenagers, analogous neural mechanisms might explain alcohol "blackouts" — a lack of memory for events that occur during a night of heavy drinking without a loss of consciousness. Blackouts were once thought to be a symptom of advanced adult alcoholism, but researchers have recently discovered just how frequent they are among teenagers as well.

Image Toren Volkmann has written a book about his problems with alcohol, which began at age 14. Credit... Sandy Huffaker for The New York Times

In a 2002 e-mail survey of 772 Duke undergraduates, Dr. White and Dr. Swartzwelder found that 51 percent of those who drank at all had had at least one blackout in their drinking lifetimes; they reported an average of three blackouts apiece.