The effects of follicle-stimulating hormone (FSH) and testosterone in rat Sertoli cells were investigated in vitro by means of isolated cell populations. The Sertoli cells selectively bind FSH, and respond to FSH stimulation with increased accumulation of endogenous cyclic AMP and secretion of androgen-binding protein (ABP). FSH binding and cyclic AMP response in the Sertoli cells change dramatically during sexual maturation. Cyclic AMP response decreases despite an increase in FSH-binding receptors per cell. Evidence has been provided for the existence of cytoplasmic and nuclear androgen receptors and chromatin acceptor-sites that specifically bind the androgen-receptor complex in the Sertoli cells. A model has been proposed for the hormonal interactions in the seminiferous tubule and the possible role of Sertoli cells in mediating the hormonal effects on spermatogenesis.