Testosterone and progesterone titers were determined by RIA in the plasma of pregnant rats and their male and female fetuses from day 17 of gestation through the day of birth and in male and female neonates on days 3 and 5 post partum. Males had significantly higher mean testosterone levels than females from day 18 of gestation through day 5 post partum. Sex differences in plasma testosterone concentrations were greatest in the fetuses on days 18 and 19 of gestation when testosterone levels peaked in the males. Instances in which female fetuses had testosterone titers equal to or greater than their male littermates were found on every day of gestation except day 18. Mean testosterone concentrations in plasma of female fetuses were high throughout gestation (greater than 1000 pcg/ml). Testosterone concentrations decreased in both sexes after birth. Differences between the sexes remained significant, and although there was an overlap in the values for males and females, testosterone concentrations in females exceeded those of their male littermates in only one out of nine pairs of samples on day 5 and in none of seven pairs on day 3 post partum. There were no significant differences in progesterone levels in plasma of males and females, either pre- or postnatally. Progesterone titers changed as a function of days post conception in both the fetuses and their mothers. In the fetuses, progesterone levels declined progressively from day 18 post conception through the day of birth, while in the mother they rose from days 18 to 19 then declined between days 20 and 21 of pregnancy. Fetuses had lower progesterone titers than their mothers. From these data, we conclude that day 18 and possibly day 19 post conception represent a critical period during which the central nervous system of the male is primed by high levels of testosterone. Thereafter, the process of masculinization is completed by exposure to testosterone levels that are relatively low and need not be consistently higher than those of female littermates.