In boys and young men, cancer therapeutic agents result in prolonged or permanent azoospermia. Preservation of these patients' fertility requires treatments that maintain endogenous stem cells and their differentiation and, in cases of total stem cell loss, autologous spermatogonial transplantation. In a rat model we proved that in-radiation or chemotherapy produced azoospermia by impairing the testicular somatic function resulting in a failure in germ cell differentiation. However, transient suppression of testosterone (T) and FSH with GnRH-antagonist (-ant) after radiation maintained or restored spermatogenesis and fertility in these rats. Similarly, in irradiated mice and monkeys, GnRH-ant treatment improved recovery of spermatogenesis from both endogenous and transplanted spermatogonial stem cells (SSC). We hypothesize that in irradiated monkeys and mice, T-suppression is most important for enhancing colonization by transplanted SSC but that maintenance of FSH levels would promote their development. Using an irradiated rhesus macaque model relevant to pediatric cancer patients, we will assess the effects of FSH maintenance during hormone suppression on spermatogenic recovery from transplanted cryopreserved germ cells from prepubertal monkeys, as well as from endogenous SSC. To more clearly define the role of T and FSH and extend results to chemotherapy, we will assess the colonization and/or spermatogenic recovery from SSC in prepubertal monkey and mouse testis cells transplanted into irradiated and busulfan- treated mouse recipient testes during the GnRH treatment, independently modulating T and FSH. Since there appears to be progressive loss of putative stem spermatogonia shortly after radiation in these two species, we hypothesize that these are indeed functional stem cells and that hormone suppression does indeed stimulate the recovery of spermatogenesis, partly by preventing the progressive loss of these surviving SSC. We will characterize these surviving putative SSC in irradiated monkeys and mice using kinetics and SSC markers, determine their viability and functionality by transplantation into recipient mouse testes, and test whether hormone suppression of the donor prevents the loss of those cells.