The monkey studies, it is hoped, will add to the effort to discern which aspects of brain function are involved in adolescent depression. In monkeys, depression is difficult to diagnose because, obviously, the animals cannot talk about feelings. Anxiety, though, is fairly easy to measure by observing behavior.

The kinds of anxious behavior the monkeys demonstrate -- not only inhibition when facing a stranger but also increased fearfulness and vigilance -- are like the actions of children with anxiety, Dr. Cameron said. In children, anxiety and depression are closely related and are thought to arise from the same biological mechanisms.

''Now, we can do all sorts of studies with the anxious monkeys that are harder to do in humans,'' Dr. Cameron said. ''We can observe their behavior every day. We can take blood samples and look at their hormones. We can assess how they respond to stress. We can ask what's different about their brains. And ultimately, we can search for genes that underlie their anxiety.''

Dr. Cameron and her colleagues are careful to say they do not believe adolescent depression is strictly a biological disorder.

''It may begin with a genetic liability, but when the early episodes of depression happen, it probably has to do with psychosocial stressors,'' said Dr. Neal D. Ryan, a psychiatrist and a leader of a 20-year study of adolescent depression at the University of Pittsburgh.

The kinds of stress that can bring about depression in adolescence, psychiatrists have found, can include both negative experiences and the lack of positive experiences -- the death of a family member, for example, and perhaps a dearth of love, warmth or support from others.

''We'd like to understand the biological piece not because we think it's all of the story or even most of the story,'' said Dr. Ronald E. Dahl, a psychiatrist and an investigator for the Pittsburgh study. ''It's because there probably is this interaction between biological systems and stress; we'd like to get insights into the brain systems involved.'' The ultimate goal is to find ways to nip depression in the bud.