Concentrations of testosterone (T) and 5 alpha-dihydrotestosterone (DHT) were measured in plasma collected from male and female ferrets at eight perinatal ages, spanning the period when behavioral sexual differentiation occurs in this species. Concentrations of T were significantly higher in males than in females 5 days before birth (day -5) and on postnatal days 10, 15, and 40. Plasma concentrations of DHT were equivalent in both sexes at all ages. In males, mean plasma T (2,278 pg/ml) and DHT (1,989 pg/ml) concentrations were highest on day -5, and declined significantly by postnatal day 5. In females, plasma concentrations of T (1,220 pg/ml) were highest on the day of birth, whereas concentrations of DHT (1,896 pg/ml) were highest on day -5; both declined significantly by postnatal day 5. The mean concentrations of T and DHT in sera from reproductively active adult male ferrets were 26,019 and 888 pg/ml, respectively, whereas sera from seasonally quiescent males contained 2,976 pg/ml T and 252 pg/ml DHT. The results demonstrate that circulating concentrations of T are significantly higher in male than in female ferrets at those neonatal ages when, in other experiments, T administration to females permanently augmented their ability to display masculine coital behavior in adulthood.