To examine the effects of follicle-stimulating hormone (FSH) on cell proliferation and cell division in ovaries, deoxyribonucleic acid (DNA) polymerase activities and DNA concentrations were determined in ovarian follicles of immature, hypophysectomized, estradiol-treated rats perfused in vitro. Ovine FSH (2-20 μg/mL) significantly enhanced the DNA polymerase a activity in the follicles perfused for 180-300 minutes, whereas the DNA polymerase β activity and DNA concentration showed no significant changes. It is concluded that FSH causes the induction of DNA polymerase at before DNA synthesis during follicle growth in immature rat ovaries.