The effects of β-carotene and vitamin A on progesterone production by cultured granulosa cells of cows were studied. The granulosa cells were isolated from medium-sized follicles (8-10mm) by the needle aspiration. When granulosa cells were treated with 50μg/dl of β-carotene for 4 days, progesterone production was increased slightly. In contrast, treatment with the same amount of vitamin A was associated with a significant (P<0.01) increase in progesterone production of the cells. Stimulation of progesterone production by vitamin A was noted in dose-dependent manners. Proliferation of granulosa cells was suppressed by vitamin A treatments, and some morphological changes were observed in cultures grown in media with vitamin A. These results suggest that β-carotene, mediated by vitamin A may stimulate steroidogenesis of granulosa cells.