The present studies examined the patterns of circulating testosterone (T) within 0-24 h after birth in male and female ferrets along with concomitant changes in neural aromatase activity and hypothalamic concentrations of estradiol (E2). Plasma and brain samples were obtained 0 and 2 h (cesarean delivery) or 0, 2, 12, and 24 h (natural delivery) after birth. Plasma T levels were significantly higher in male neonates 2 h after birth than at 0 h in both cesarean-delivered (9.48 +/- 1.25 vs. 3.37 +/- 0.60 ng/ml) and naturally delivered (19.28 +/- 2.94 vs. 5.13 +/- 1.93 ng/ml) ferrets, while female neonates showed no significant changes in T over these sampling times. T levels had returned to 0 h levels by 12 h in naturally delivered males. T was significantly lower in females than in males 0, 2, and 24 h after natural delivery, whereas T levels were equivalent in males and females immediately after cesarean delivery. Male kits kept on a heating pad for 2 h after natural delivery had lower plasma T levels than males that were left with their mothers over this same period. Brain aromatase activity in anterior hypothalamus-preoptic area, medial basal hypothalamus (MBH), temporal lobe, and cerebral cortex was equivalent in males and females at all postpartum ages, regardless of whether delivery occurred by cesarean section or naturally. However, in naturally delivered kits of both sexes significant elevations in aromatase activity occurred in MBH and temporal lobe 24 h postpartum. Finally, E2 concentrations in anterior hypothalamus-preoptic area and MBH were equivalent 0 and 2 h postpartum in males and females, regardless of whether they were delivered naturally or by cesarean section. The observed postnatal elevation in T may contribute to brain and behavioral sexual differentiation of male ferrets. It is unclear, however, whether such an effect of T depends on its neural aromatization to E2.