Aim: To assess proliferative and apoptotic potential of the seminiferous epithelium cells in relation to Sertoli cell maturation in newborn rats under the influence of estradiol, follicle stimulating hormone (FSH) or both agents given together. Methods: From postnatal day (PND) 5 to 15 male rats were daily injected with 12.5 μg of 17β-estradiol benzoate (EB) or 7.5 IU of human purified FSH (hFSH) or EB + hFSH or solvents (control). On postnatal day 16, autopsy was performed. Sertoli cell maturation/function was assessed by morphometry. Proliferation of the semini- ferous epithelium cells was quantitatively evaluated using immunohistochemical labeling against proliferating cell nuclear antigen and apoptosis using the TUNEL method. Results: Although EB inhibited Sertoli cell maturation and hFSH was not effective, a pronounced acceleration of Sertoli cell maturation occurred after EB + hFSH. Whereas hFSH stimulated Sertoli cell proliferation, EB or EB + hFSH inhibited Sertoli cell proliferation. All treatments signifi- cantly stimulated germ cell proliferation. Apoptosis of Sertoli cells increased 9-fold and germ cells 2-fold after EB, and was not affected by hFSH but was inhibited after EB + hFSH. Conclusion: At puberty, estradiol inhibits Sertoli cell maturation, increases Sertoli and germ cell apoptosis but stimulates germ cell proliferation. Estradiol in synergism with FSH, but neither of the hormones alone, accelerates Sertoli cell maturation associated with an increase in germ cell survival. Estradiol and FSH cooperate to induce seminal tubule maturation and trigger first spermatogenesis. (Asian J Androl 2008 Jul; 10: 585-592)