Treatment of male rat pups from five to 34 days of age with dihydrotestosterone or 5 alpha-androstane-3 alpha, 17 beta-diol, resulted in reduced testicular size at 35 days of age. This appeared to be due to decreased tubular diameters and reduced spermatocyte numbers, especially late pachytene cells in stages X to XIII. In rat pups treated with dihydrotestosterone or 5 alpha-androstane-3 alpha, 17 beta-diol, treatment with luteinizing hormone resulted in some restoration of tubular diameter and spermatocyte numbers. In 5 alpha-androstane-3 alpha, 17 beta-diol treated rat pups partial restoration of testes size resulted from follicle-stimulating hormone treatment, but tubular diameter and spermatocyte numbers were depressed. Single serum samples collected at 35 days of age were analysed for luteinizing and follicle-stimulating hormone concentrations. These data indicated that the effects of administered androgens were due primarily to depression of circulating gonadotropin concentrations but a direct inhibition at the level of the testis could not be ruled out.