During restraint stress, 30-day-old rats uniformly show an abrupt and persistent decrease in activity with a corresponding increase in quiet wakefulness. However, the effect of restraint on sleep and body temperature depends on whether maternal separation had previously occurred at the customary age (day 22) or 7 days earleir (day 15). If maternal separation took place on day 22, subsequent restraint on day 30 has no effect on the amount of time spend in sleep or on body temperature, relative to a comparison sgroup of unrestrained rats. But if maternal separation took place on day 15, restraint on day 30 elicits a marked initial increase in sleep and a later decrease in sleep and body temperature. The results are consistent with the interpretation that premature maternal separation retards the normal maturation of these behavioral and thermoregulatory responses to restraint stress.