Aromatase activity is higher in the male than in the female anterior hypothalamic-preoptic area (POA) in both the avian and the rodent adult brain. This sex difference is abolished after castration of the male and restored by androgen treatment. Gonadectomy has no effect on POA aromatase in the female. The aim of this study was to find out whether sex dimorphism in adult POA aromatase is only due to a sex difference in circulating gonadal hormones or dependent upon sexual differentiation of the brain. Aromatase activity was measured in vitro in microdissected POA samples using a sensitive radiometric assay. We examined the effects of gonadectomy and testosterone treatment on enzyme activity in adult rats and doves of both sexes. We also studied the effects of neonatal gonadectomy and hormone substitution in male and female rats. The results suggest that levels of POA aromatase in the adult depend primarily on gonadal activity, but that mechanisms involved in the regulation of aromatase activity and enzyme induction may be sex-specific and could result from sexual differentiation of the brain in early life. Further work will be required to determine the developmental stage when this occurs and the exact mechanism(s) responsible for increased sensitivity of the adult male POA to the inductive effect of testosterone.