Spermatogenesis is one of the most productive self-renewing systems in the body: on the order of 10⁷ spermatozoa are produced daily per gram of testis tissue. In each mammalian species, the time required for completion of the process is unique and unalterable. Because the process is supported by somatic Sertoli cells, it has generally been thought that cell-cell interaction between germ and Sertoli cells controls the duration of cell cycles and cellular organization. We have used the newly developed technique of spermatogonial transplantation to examine which cell type(s) determines the rate at which germ cells proceed through spermatogenesis. Rat germ cells were transplanted into a mouse testis, and the mouse was killed 12.9–13 days after administration of a single dose of [³H]thymidine. The most advanced rat cell type labeled was the pachytene spermatocyte at stages VI–VIII of the spermatogenic cycle. In animals given only rat cells, some endogenous spermatogenesis of the mouse recovered. The most advanced labeled mouse cell types in recipients killed 12.9–13 days after administration of a single dose of [³H]thymidine were meiotic cells or young spermatids, which is consistent with a spermatogenic cycle length comparable to the 8.6 days reported for the mouse. The same results were obtained if a mixture of rat and mouse cells were transplanted. There existed two separate timing regimens for germ cell development in the recipient mouse testis; one of rat and one of mouse duration. Rat germ cells that were supported by mouse Sertoli cells always differentiated with cell cycle timing characteristic of the rat and generated the spermatogenic structural pattern of the rat, demonstrating that the cell differentiation process of spermatogenesis is regulated by germ cells alone.