Serum concentrations of testosterone, oestrogens, luteinizing hormone and follicle-stimulating hormone in male and female rats during the critical period of neural sexual differentiation.

Untreated male and female rat pups were killed 1--5 days post partum and the serum concentrations of testosterone, oestrogens, LH and FSH were determined by radioimmunoassay. At all five sampling times, the serum concentrations of testosterone in male rats were about three times higher than those in female rats, but serum levels of oestrogens did not differ between the sexes. Serum concentrations of LH and FSH were lower in male than in female pups. In another study, rats were decapitated 1--10 days after birth and serum concentrations of testosterone were determined with a different radioimmunoassay. Again, at all four sampling times, the concentration of testosterone was significantly higher in the male than in the female pups.