Testicular biopsies were obtained at various intervals after intratesticular injections of tritiated thymidine. Radioautographs revealed that the most mature labeled germ cells were the preleptotene spermatocytes at 1 hour, the midpachytene spermatocytes at 16 days, and the immature spermatids at 32 days after injection. These results indicated that one cycle of the seminiferous epithelium lasts 16 days, and the whole of spermatogenesis is estimated to consume approximately 64 days.