Pregnant rats were passively immunized against testosterone by injections of rabbit antibodies against testosterone. Fetuses were studied daily starting at day 18 of gestation. Testosterone antibodies were detected in all fetal plasma samples. At day 18 of gestation testicular testosterone content – as an index of gonadotropin stimulation – was unchanged even though circulating free testosterone was reduced by antibody binding. However, at day 19 and all subsequent days of gestation testicular testosterone content was significantly higher in immunized fetuses compared to the controls. These results indicate that in the male rat the negative feedback between testes and the hypothalamopituitary system develops between days 18 and 19 of gestation.