Aromatase activity in the brain of the rat was measured during perinatal development and in adulthood. In pooled hypothalamus, preoptic area, septum and amygdala (HPAS), estrogen formation per gram protein increased between embryonic days 15 and 19 and then declined. Results for males and females were indistinguishable, except for post-natal days 1 and 2 and 3 and 4 when estrogen formation was significantly higher in the male. Aromatization was not convincingly demonstrated in cerebral cortical tissue at any time studied. Regional dissection of the HPAS in 2–3 day post-natal animals revealed significant sex differences in aromatase activity in the corticomedial amygdala and the hypothalamus, but not in the preoptic-septal region or the remainder of the amygdala. These results are consistent with the idea that perinatal androgen secretion in the male rat may stimulate estrogen formation in the brain.