Neonatal and prepubertal male rats were treated with human chorionic gonadotropin (hCG, 5 IU/g body weight per day) on days 2-4 or 20-22. Depending on the date of treatment, different groups of rats were sacrificed at 5, 23, 30 and 100 days of age, in order to study the short- and long-term effects of the treatment with hCG on the development of the testes and sex accessory organs. Rats treated with hCG on days 2-4 showed increased number and size of foetal Leydig cells at 5 days of age. However, long-term effects include decreased numbers of adult-type Leydig cells, decreased weight of the testes and sex accessory organs, decreased basal and hCG-stimulated testosterone secretion, and delayed balano-preputial separation. In contrast, animals treated with hCG on days 20-22 showed similar short- and long-term effects, consisting of increased number of adult-type Leydig cells and macrophages, increased weight of the testes and sex accessory organs and advanced balano-preputial separation. In adulthood, both groups showed normal reproductive function. These results seem to indicate that the effects of hCG treatment in prepubertal rats are dependent on the type of Leydig cell stimulated, and suggest that foetal Leydig cells play a regulatory role in the early postnatal testicular development.