Female sexual behavior in male rats: effect of hour of castration at birth.

In the male rat, a dramatic increase in serum testosterone occurs during the first four hours of postnatal life. The experiments sought to determine whether such an increase would participate directly on the defeminization process. Newborn male rats were castrated either at 0 hr in utero (literally at the moment of birth) or at 6 or 12 hrs after birth. Some males were castrated at 0 hr in utero and injected at the time of surgery with 1 or 5 micrograms testosterone propionate (TP). At about 90 days of age, each animal was injected with estrogen and progesterone and tested for female sex behavior. Males castrated at 0 hr in utero displayed typical female sex behavior. Males castrated at 6 or 12 hrs after birth were less receptive than males castrated at 0 hr. Males castrated at 0 hr and injected with testosterone at this time almost never showed lordosis as adults after treatment with ovarian hormones. These results are consistent with the idea that the rapid elevation in serum testosterone which occurs shortly after birth suppresses the development of sexual behavior sensitivity to ovarian hormonal stimulation.