Cabergoline (CG), a dopamine agonist, and prolactin (PRL) added in vitro to isolated granulosa cells from immature rats cultured in serum-free medium inhibited FSH-stimulated production of progesterone. The lowest effective concentrations of CG and PRL were 10(-8)M and 1 ng/mL, respectively. In combination, the inhibitory activities were additive. CG at 10(-8) M also suppressed estradiol production by isolated rat granulosa cells; whereas PRL did not. In conclusion, CG inhibits FSH-stimulated progesterone and estradiol production by rat granulosa cells.