Male rats were daily injected with human FSH (hFSH) or estradiol benzoate (EB) or hFSH+EB between day 5 and 15 of life and autopsied on day 16. hFSH accelerated testicular growth, increased number of spermatogonia and serum level of testosterone. hFSH stimulated also spermatogonia differentiation, which resulted in 5-fold increase of the number of spermatocytes. EB given alone induced adverse, inhibitory effects on spermatogenesis and serum testosterone, did not influence serum FSH and LH but increased 14-fold the level of prolactin. Except from testosterone, EB given with hFSH not only overcame inhibitions, but multiplied hFSH stimulatory effects on spermatogenesis up to 30-times of control values. In addition, after FSH+EB premeiotic germ cell ratio reached adult type value precociously. Estradiol may play regulatory roles in testicular maturation (1) inhibitory, direct one or resulting from decrease in testosterone secretion; (2) stimulatory, by enhancement of FSH action, with a possible involvement of prolactin that may act in concert with FSH.