The response of ovarian weight, uterine weight, and enzyme activity of succinic dehydrogenase (SDH) to chorionic gonadotrophin (HCG) and serum gonadotrophin (PMS) treatment at various ages of the developing rat were compared. The greatest response was found at the age of about 30 days. While the uterine weight increases to the same extent as a result of HCG and PMS treatment at all ages examined, the ovarian weight increase and the enzyme activity of SDH are different for these two gonadotrophins. The possible reasons for the maximal reactivity at the age of 30 days are discussed.