Cell reorganization experiments in vitro were performed with dissociated rat testes at different ages of postnatal development namely, newborn, 8-10, 18-25, 35-40, and 90 days. Only newborn and juvenile rat testicular cells reassociated into testicular-like organization in rotation culture. Puberal and adult rat testicular cells show morphogenetic organization when they were deprived or germ cells by busulphan pretreatment. A factor present in testicular tissue of puberal and adult rats inhibits reorganization. The inhibitor is confined to the spermatic cell fraction in the testis.