Changes of serum gonadotropin concentrations in fetal and neonatal rat following castration

Changes of serum concentrations of LH were measured in fetal and neonatal rats following castration. Intact fetal and neonatal male rats showed low levels of serum LH concentration. Though serum LH levels of male rats casterated on the 20th day of gestation did not significantly increase by the 22nd day of fetal age, serum levels of neonatal male rats increased 2-to 3-fold 3 days following castration at all ages studied. The increase was greater in 4-day-old male rats than in 1-day-old males. The increase of serum LH after castration was also found in 4-day-old male rats castrated on the 20th day of gestation. In contrast, serum levels of 1-day-old neonatal female rats did not increase 3-days after castration. These observations suggest sex differences in maturation of the gonadal-hypophyseal feedback mechanism.