: OBJECTIVES: It has been reported that GHRH-GH-IGF-1 system plays an important role in the regulation of ovarian follicular development and maturation. METHODS: In order to evaluate the direct effects of growth hormone releasing hormone (GHRH) and growth hormone (GH) on steroidogenesis, the effects of GHRH and GH on progesterone and estradiol release from cultured rat granulosa cells were examined. The progesterone and estradiol in supernatants were measured with RIA methods. RESULTS: Our results demonstrated that the addition of GH to the culture medium produced a marked stimulation of progesterone and estradiol. The stimulating effects were observed after administration of GH in all concentrations: 1, 10, 100 nM during 60 and 120 mins of incubation. During 240 mins of incubation the minimal stimulation of progesterone and estradiol was found. However, GHRH administered in 1, 10 and 100 nM did not change progesterone and estradiol release from cultured granulosa cells. CONCLUSION: Growth hormone (GH) but not GHRH has direct stimulating effects on progesterone release from cultured rat granulosa cells.