Spermatogenesis is inherently a highly organized, efficient and complex process controlled by cellular and hormonal interactions that are not well understood. In order to comprehensively investigate TESTIS function, an in vitro or in vivo model for spermatogenesis is required that is accessible and can faithfully mimic the donor tissues environment. TESTIS tissue xenografting involves grafting of small fragments of TESTIS tissue from immature donor males of a variety of mammalian species under the back skin of host mice to allow the grafted tissue to grow and develop. TESTIS tissue xenografting has numerous potential applications. Perhaps most importantly, it can be used as an in vivo culture system to study TESTIS function from a variety of donor species in a laboratory mouse. Using this culture system, virtually all aspects of TESTIS function remain intact and the TESTIS tissue is accessible for study and interventions. Therefore, it can serve as a unique and previously unavailable in vivo system for the study of spermatogenesis and steroidogenesis in a controlled manner not feasible in the target donor species. Completion of primate spermatogenesis in TESTIS tissue fragments originating from prepubertal donor monkeys grafted into immunodeficient mice shows this technique can be used to examine spermatogenesis of primates and possibly humans, and can potentially be used as an in vivo system to study the endocrine/paracrine control of human TESTIS in a laboratory model. This for example can be used to test the in vivo effects of new male contraceptives in the form of candidate compounds or promising hormone-therapy regimens directly on miniature primate testes incubated inside a mouse.