The effects of chronic, mild stress and chronic narcotic antagonism during maturation of mice on the sensitivity to pain and to the analgesic effect of morphine were examined. Analgesia was measured using the tail-flick assay. Chronic stress, produced simply by the subcutaneous injection of mice twice daily with saline starting day 5 postpartum, produced an increase in the sensitivity of these mice, when mature, to the analgesic effect of morphine. A similar schedule of saline injections for 4 weeks in adult mice did not alter the sensitivity of those mice to the analgesic effects of morphine. Chronic injections of narcotic antagonists during maturation did not produce an effect on morphine analgesia different from that after chronic injections of saline. However chronic exposure to narcotic antagonists since conception, compared to chronic injections since day 5 postpartum in the offspring, did produce differential effects on the control tail-flick latencies such that naloxone during gestation decreased, while naloxone during gestation and postpartum increased tail-flick latencies. These data suggest that exposure to chronic stress during early development is responsible for an altered sensitivity to narcotic analgesics while exposure to naloxone during maturation affects pain perception.