Hormone treatments that suppress sperm production enhance the recovery of spermatogenesis after gonadal exposure to various cytotoxic agents. It has generally been assumed that the mechanism of protection involved an arrest of spermatogonial kinetics. To test this hypothesis critically, we examined spermatogonial kinetics and numbers in rats in which the completion of spermatogenesis was suppressed with a 6-week testosterone plus 17beta-estradiol treatment that protected the testis from procarbazine-induced damage. Histological examination showed that the numbers of A-aligned, intermediate, and B spermatogonia and preleptotene spermatocytes and their mitoses were unaffected by testosterone plus 17beta-estradiol treatment. Flow cytometric analysis of bromodeoxyuridine-labeled cells showed that the percentage of diploid cells undergoing DNA synthesis, the progression of B spermatogonia and preleptotene spermatocytes through S-phase, the division of intermediate and B spermatogonia, the entry of intermediate spermatogonia into their next S-phase as type B cells, and the progression of cells through meiotic prophase were either unchanged or very slightly increased. Thus, changes in spermatogonial numbers or suppression of their proliferation cannot account for protection of spermatogenesis from exposure to cytotoxic agents.