The aims of the study were to compare the in vitro effects of daidzein or 17beta-estradiol (E(2)) on: 1) progesterone (P(4)) secretion by luteinized granulosa cells harvested from large porcine follicles, as well as 2) estrogen receptor alpha and beta (ERalpha and ERbeta) mRNA and protein expression in the cells. In addition, the effect of daidzein on E(2) secretion and viability of the granulosa cells was examined. We found that basal and gonadotropin-stimulated P(4) secretion were inhibited in granulosa cells cultured in the presence of daidzein either for 24 or 48 hours. In contrast to daidzein, E(2) reduced P(4) secretion only during 24-hour cell cultures increasing it during longer cultures. Daidzein did not affect E(2) secretion by granulosa cells. The expression of ERalpha and ERbeta mRNA, as well as ERbeta protein, was up-regulated by daidzein but unaffected by E(2). To conclude, the soy estrogen daidzein acts directly on the porcine ovary to decrease progesterone production and to increase expression of ERbeta mRNA and protein. Daidzein actions in porcine luteinized granulosa cells differ from those of estradiol and it may suggest disadvantageous effects of the phytoestrogen on reproductive processes in females.