Immature Sertoli cells proliferate and several factors affect their number, including the follicle stimulating hormone (FSH), testosterone, estradiol and several paracrine growth factors. Using a primary culture of Sertoli cells isolated from 15-day old Wistar rats we have shown that relaxin stimulates Sertoli cell proliferation through the activation of MEK/ERK1/2 and PI3K/AKT pathways. In contrast, FSH inhibited both ERK1/2 and AKT phosphorylation. Furthermore, FSH strongly increased cAMP production, whereas relaxin inhibited basal cAMP production. Our results indicate that in rat Sertoli cells from 15-day old rats relaxin and FSH affect the same signaling pathways in opposite directions. Interplay between both hormones may be important to control the proliferation and differentiation of Sertoli cells.