The present study examined the role of progesterone in regulating human granulosa cell proliferation. Human granulosa and luteal cells were obtained from follicular aspirates of women undergoing in vitro fertilization. Cells were plated at 5, 10, or 50 x 10(3) cells/mL and cultured for up to 6 days. At specific times, cells were harvested and assessed for cell number and morphology. The medium was assayed for progesterone. Cells plated at 50 x 10(3) cells/mL did not increase in number after 3 or 6 days of culture, but rapidly differentiated, secreting high amounts of progesterone (> or = 320 nmol/L). Conversely, cells plated at 5 x 10(3) or 10 x 10(3) cells/mL doubled in number over the first 3 days of culture and subsequently differentiated. The addition of 100 ng/mL progesterone or more to the medium inhibited proliferation. Aminoglutethamide blocked progesterone secretion and increased the number of cells present after 3 days of culture. The antiproliferative effects of progesterone were not mimicked by estradiol, testosterone, dihydrotestosterone, or dexamethasone and could not be overridden by epidermal growth factor, a potent mitogen. These data suggest that progesterone plays an autocrine/paracrine role in regulating granulosa cell proliferation.