Male and female mice and hamsters were decapitated 1-5 days after birth and serum concentrations of testosterone determined by radioimmunoassay. In the two species studied, serum levels of testosterone in male pups were significantly (P less than 0.05) higher than those obtained in female neonates. This lends support to the hypothesis that circulating levels of testosterone play an important role in the process of neural sexual differentiation in rodents. Moreover, the sex differences in serum concentrations of testosterone in neonatal rodents together with the detectable levels of testosterone in female neonates may suggest that androgenization is a dose-dependent phenomenon. Alternatively, they may indicate that a minimum concentration of the steroid must be present for androgenization to occur during the critical period of neural sexual differentiation and that this 'threshold' is exceeded in male but not in female rodents.