Testosterone formation was measured in fetal and neonatal (2 h to 20 days) rat testes incubated with and without 100 IU human chorionic gonadotrophin (hCG). At all ages studied, testosterone production was significantly elevated in the presence of hCG. Neonatal testes showed an absolute drop in testosterone production with increasing age, but the percentage increase above baseline produced by hCG stimulation leads to the speculation that this decrease is due to a relative reduction in the number of Leydig cells with increasing age rather than to a lowered sensitivity to hCG stimulation.