Accurate assessment of gonadal steroid levels in the developing brain is critical for understanding naturally occurring steroid-mediated sexual differentiation as well as determining the physiological relevance of exogenous steroid treatments commonly used in the study of this phenomenon. Using RIA, we measured the estradiol (E2) content of six regions of the developing brain immediately post partum, 1 d post partum, and after injection of exogenous estradiol benzoate, testosterone propionate, or the aromatose inhibitor formestane. We found sexually dimorphic E2 content in several regions of the newborn brain. At 2 h of life, there was significantly higher E 2 content in males vs. females in the frontal cortex, hypothalamus and preoptic area but not in the hippocampus, brainstem, or cerebellum. Surprisingly, the female hippocampus had significantly higher E2 content than all other female regions examined. By d 1 postpartum, E2 levels had decreased precipitously in most brain regions, and only the hypothalamus maintained a sex difference. Injection of female pups with estradiol benzoate raised tissue levels to that of the male in the hypothalamus but 2- to 3-fold higher in the other five regions. Testosterone administration increased E2 content exclusively in the preoptic area, suggesting local variation in aromatase activity and/or substrate availability. Central administration of formestane decreased estrogen content in the male cortex, hypothalamus, and preoptic area. Formestane treatment also decreased endogenous E2 in female cortex and hippocampus, suggesting de novo synthesis selectively in these brain regions. These data corroborate and extend previous findings of sex differences in brain E2 levels perinatally and reveal an unexpected regional heterogeneity in E2 synthesis and/or metabolism.