[Aromatase activity by the diencephalon of fetal and neonatal rats].

Aromatase activity in the diencephalon was investigated during the perinatal period in which sex differentiation occurs critically in the rat. 1) Fetal plasma testosterone levels at 17 and 20 days of gestation were slightly higher in the male than in the female. During the critical period after delivery, the hormone levels in the male neonates increased markedly, while it decreased rapidly in the female. 2) Aromatase activity was followed in the diencephalon by measuring the production of estradiol from a labelled substrate, 4-14 C-testosterone. Considerable enzyme activity was detected at 14 days of gestation and it decreased slightly thereafter. The enzyme activity, however, increased drastically during the postnatal critical period. After that it decreased again in both male and female neonates. 3) Aromatase in the telencephalon showed very weak activity with a fluctuation similar to that in the diencephalon. These results suggest that estrogens in the male pups which are specifically converted from testosterone during the critical period may be decisive in sex differentiation of the central nervous system in the rat.