Adult rams were hypophysectomized and treated for 20 days with testosterone (2 X 0.25 g/day), PMSG (2 X 300 i.u./day) or hCG (2 X 250 i.u./day), or for 40 days with testosterone (2 X 0.25 g/day). All treatments maintained a normal concentration of testosterone within the seminiferous tubules. Quantitative histological analysis showed that (1) the differentiation from A0 to A1 spermatogonia was maintained by PMSG or hCG but not completely by testosterone; (2) the transition from intermediate spermatogonia to primary spermatocytes was maintained only by PMSG but not by testosterone or hCG; (3) meiotic prophase and spermiogenesis were maintained by the three hormones but there were qualitative abnormalities in the spermatids. These results suggest that in the ram, the differentiation of renewing stem spermatogonia is under LH control and that the last stages of spermatogonial multiplication, from intermediate to B spermatogonia and to primary spermatocytes, are under the control of the FSH-like activity of PMSG.