Male rats castrated before puberty (when 26 days of age) showed a progressively decreasing susceptibility to the inhibitory effects of morphine (5 mg/kg) upon LH secretion for up to 28 days after gonadectomy (approximately 100%, 40% and 10% inhibition at 5, 12 and 28 days after castration), but thereafter morphine again caused approximately 50% reduction in serum LH values; the minimum inhibition found at 28 days after castration (age 54 days) occurred at the time at which male rats normally reach puberty. When rats were castrated at 59 days of age, morphine maximally suppressed serum LH concentrations (to less than 70%) 2 and 5 days after castration, but had no effect thereafter. In prepubertal castrates, testosterone replacement between Days 26 and 50 of life resulted in responses to morphine similar to those found in rats castrated after puberty, i.e. serum LH levels were not reduced. Morphine significantly reduced LH levels in prepubertal castrates given testosterone after 60 days of age. Treatment with morphine consistently elevated serum prolactin concentrations (greater than 100%) in castrated rats of all ages, regardless of the time elapsed after gonadectomy. These results indicate a transient fall in the inhibitory opioidergic tone upon LH secretion as the normal age of puberty approaches, that the ability of opiates to alter LH release in adulthood may depend upon testicular steroids secreted during the peripubertal period, and that the LH responses do not reflect general changes in the neuroendocrine response to opiates after castration since the prolactin response to morphine remains intact in rats castrated before and after puberty.