Morphine exposure during development has been shown to produce fetal tolerance to morphine as measured by spontaneous activity only if a particular injection schedule is used. The present study was undertaken to compare the morphine-induced analgesic response in adult offspring of rats which had been injected during the last half of gestation on schedules known to produce fetal tolerance (5 mg/kg morphine at 6 hour intervals) versus a schedule known not to produce fetal tolerance (10 mg/kg morphine at 12 hour intervals). At 30 days postnatally the offspring of animals injected on these 2 schedules show no changes in their responsiveness to the analgetic effect of morphine as determined in the hot-plate test. The present study shows that adult offspring of mothers injected with 20 mg/kg/day of morphine in four divided doses on days 12-20 of gestation have an enhanced analgetic response to morphine in the tail-flick test. In contrast, offspring of mothers injected during the same period of gestation with 20 mg/kg/day of morphine in two divided doses respond to the analgetic effect of morphine in the same manner as the offspring of saline-treated mothers. These results show that the schedule for prenatal morphine administration can play a role in the behavioral effects of morphine in adulthood.