Prenatal and early postnatal sex differences in plasma and gonadal testosterone and plasma luteinizing hormone in female and male rats.

Plasma levels of testosterone and LH were estimated in female and male rats at gestational ages of 19, 20 and 21 days and at 1, 3, 6, 12, 18 and 24 h after birth. Concentrations of testosterone in the gonads were also estimated in 20-day-old fetuses and at various times after birth. Before birth female fetuses had significantly lower levels of testosterone and higher levels of testosterone and higher levels of LH than had male fetuses. During the first 24 h after birth female rats also had lower levels of testosterone and higher levels of LH than male rats. The pattern of levels of testosterone in the hours after birth was significantly different between female and male rats in that high levels were observed 1 and 3 h after birth in male rats (3.0 and 2.2 ng/ml respectively). This finding, as well as the relatively high levels of testosterone in female fetuses (about 50% of the levels found in male womb-mates) is discussed.