Inthe adult male, germ cell differentiation takes place inthe seminiferous tubules ofthe testis by a complex, highly organized and very efficient process. A population of diploid stem-cell spermatogonia that lieon the basement membrane althe tubule continuously undergoes self-renewal and produces progeny cells. which initiate the process of cellular differentiation to generate mature spermatozoa. Each testis contains many seminiferous tubules. which are connected at both ends to acollecting system called the rete testis. The mature spermatozoa pass from the tubules into the rete and are then carried through efferent ducts to the epididymis for final maturation before they are ready to fertilize an egg. Inprevious studies, we have demonstrated that donor testis cells collected from a fertile mouse are able to generate spermatogenesis when transplanted to the seminiferous tubules of an infertile male. The spermatozoa produced by the recipient from the donor-derived spermatogonial stem cells are able to fertilize eggs and produce progeny carrying the donor male haplotype. Furthermore, donor testis stem cells from a rat will generate normal rat spermatozoa following transplantation to a mouse testis. The spermatogonial transplantation technique is clearly valuable and applicableto many species, but itisdifficult. Therefore, several procedures to introduce donor cells into the seminiferous tubules of a recipient have been developed using the mouse as a model, and they are described here in detail. The results indicate that microinjection of cell suspensions into the seminiferous tubules, efferent ducts orrete testis are equally effective ingenerating donor cell-derived spermatogenesis in recipients. Each approach is likely to be useful for different experimental purposes in a variety of species.