Imagine a test that could pinpoint which babies will be particularly susceptible to stress. And then imagine teaching these stress-prone children special skills to protect themselves throughout life.

Someday soon, this idea may be more than fantasy. Already, Stephen Suomi, chief of the laboratory of comparative ethology at the National Institute of Child Health and Human Development, can identify stress-prone rhesus monkeys only weeks old. Ongoing studies by Harvard University psychologist Jerome Kagan and others suggest that the same may be possible one day in humans.

Evidence suggests that these stress-prone children and animals perceive situations differently than their more relaxed counterparts, Suomi says. "The bottom line is that they react differently."

How an individual reacts to stress is at least partly shaped by genetics. The DNA code that dictates what color eyes a person has and whether he or she will be tall or short also seems to carry a message about behavior, studies suggest.

For instance, Suomi has found that about 20 to 25 percent of rhesus monkeys living in a breeding colony -- and perhaps of all primates as well -- can be classified as "highly reactive," or particularly susceptible to stress.

In the wild, there could be a biological advantage to being "uptight." "This extra energy, if channeled appropriately . . . may make you a better fighter in the long run, or better able to protect your infant for a longer period of time, or to look out for potential problems of this sort," Suomi says.

But at 1 to 4 weeks of age, these baby monkeys showed "poor" visual and hearing responses to tests. They were shy, had "relatively poor" muscle tone and were "delayed slightly" in motor reflex development.

"We noticed that these things could be picked out very early in life, and we now can pick them out in first couple of weeks, if not earlier," Suomi says.

By 1 to 6 months of age, when separated for two hours from their mothers, the high- reactive monkeys showed "extreme and prolonged" rises in the stress chemicals cortisol and ACTH (adrenocorticotropin hormone) compared with their calmer peers. These monkeys also were less likely to move, explore, play and make sounds during separation from their mothers.

"Our so-called high reactive infants are the last to leave their mothers," Suomi says. "They leave for shorter periods of time and they are the last to start interacting with peers . . . these highly reactive individuals seem to be somewhat shy and socially withdrawn until they get used to the situation, and then they are fine."

Environment also plays a role. High-reactive infant monkeys reared by calm surrogate mothers show a slightly blunted response to stress compared with those high-reactive infants reared by a high-reactive parent.

In related studies at Harvard, psychologist Kagan and his colleagues are studying 2- and 3-year-old children who appear to be "extremely fearful, timid and vigilant."

By studying behavior and measuring hormones in the children's saliva, Suomi reports that these children are "very much like our infant monkeys ."

Like the highly reactive rhesus monkeys, when these children are challenged by unknown circumstances, "they seem to show unusual adrenocortical activity," Suomi says. "They show differences in patterns of heart rate changes . . . Those individuals who show these physiological profiles are the same ones who are the last or the most reluctant to try these things out. If you give them enough time, they settle down and behaviorally you can't tell them from the others. So I think that we may be talking about parallel if not identical phenomenon."

About half the high-reactive children "have changed by age 7.5 years because of parental pressure and their own desires to be less timid ," Kagan says. The hope is to idenfity stress-prone children early and help them learn to cope better with the tensions and pressures of life. In turn, that may lower their risk of developing stress-related diseases such as high blood pressure, heart disease and even depression.