The effects of sustained delivery of morphine and/or testosterone (T) on male rat copulatory behavior, penile reflexes and dopaminergic metabolism in selected brain regions were examined. Castration was followed by (1) a decrease in the number of male rats exhibiting intromissive and ejaculatory behavior in mating tests, (2) decreased erections in ex copula tests, and (3) increases in dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations in the mediobasal hypothalamus (MBH) and the preoptic area-anterior hypothalamus (POA-AH). The decreased incidence of copulatory behavior and penile reflexes seen after castration was effectively prevented by a 4-day treatment with 5-mm T-containing Silastic capsules. Chronic morphine implants, conversely, accentuated the castration-induced decrements in copulatory behavior and prevented the 5-mm-T-induced facilitation, but did not alter the number of animals displaying erection (although the number of erections displayed by testosterone-treated rats was reduced) in ex copula tests. Treatment of castrated rats with 5 mm T, but not morphine alone, nor the combination of 5 mm T plus morphine, significantly reduced dopamine and DOPAC levels in the MBH. In the POA-AH, 5 mm T was without effect, whereas morphine, alone or in combination with 5 mm T, reduced the levels of dopamine and DOPAC. These data suggest that (1) the decline in sexual behavior induced by chronic morphine is primarily due to a failure of sexual arousal, and not of erectile ability, and (2) although the decline in sexual activity seen after castration is associated with alterations in dopaminergic metabolism, the effects of morphine and testosterone on sexual activity are opposite and dissociated from alterations in dopaminergic metabolism.