Duromorph, a long-acting form of morphine, was administered to pregnant golden hamsters and/or their pups over the last 4 days of pregnancy and/or the first 4 days after birth. As adults, offspring were gonadectomized, primed with estrogen and progesterone, and tested for their ability to display feminine sexual behavior when placed with a stud male. They were then given testosterone over a 4-week period and tested for their ability to display masculine sexual behavior in the presence of a receptive female. Perinatal morphine exposure had little effect on the females' ability to display either feminine or masculine sexual behavior. In contrast, feminine sexual behavior was significantly enhanced in males exposed to morphine over the perinatal period. This suggests that exposure to opiates during the critical period of sexual differentiation may prevent the defeminization process in this species.