The purpose of this study was to determine the effect that chronic maternal exposure to narcotics has on the effectiveness of analgesic drugs in the offspring. No exposure related differences were observed in the base level response to the noxious stimuli used. However, offspring of methadone and morphine-treated mothers tended to show decreased response latencies compared to control offspring, in both the hotplate and tail flick test, following the subcutaneous administration of narcotic analgesic drugs. In all groups studied, methadone offspring had significantly reduced latencies while morphine offspring had latencies that were intermediate between the control and methadone-treated groups (in general, control latencies greater than morphine latencies greater than methadone latencies). Morphine offspring were significantly different from the controls only in the 120-day-old female group. Treatment-related decreases in the effectiveness of the analgesics in both 25- and 120-day-old offspring suggest that exposure of developing rat pups to narcotics during gestation and early postnatal development is associated with long-lasting alterations in those processes involved with pain perception and/or interpretation.