Although systemically administered testosterone can effect the postnatal maturation of elements in the seminiferous tubule in hypophysectomized rats and mice, it does not elicit the same degree of development which occurs in normal control animals. In view of reports of precocious spermatogenesis in androgen secreting Leydig cell tumors, the present study was designed to determine if high local levels of testosterone accelerate development of the seminiferous tubules. Testosterone pellets were inserted under the tunica albuginea of the right testis of 7 day old rats. At 17, 23 and 28 days of age development of the seminiferous tubules, as judged by the formation of tubule lumens, was more extensive in the treated testes than in contralateral and sham operated controls. Tubule diameters were not necessarily correlated with lumen formation. This study demonstrates that high local levels of testosterone accelerate seminiferous tubule development in the rat and indicates that tubule diameter may not be a valid basis for estimating development of the testis. It is suggested that testosterone exerts this effect through its actions on the Sertoli cells.