The effect of plant-derived, nonsteroidal estrogens (genistein, daidzein and biochanin A) on cortisol and androstenedione secretion by porcine adrenocortical cells was examined. Adrenals were harvested from locally slaughtered mature gilts on days 5-9 of the oestrous cycle. Adrenocortical cells were incubated with or without genistein, daidzein, biochanin A (0.5, 5, 10 or 50 microM), oestradiol (10, 50, 100 or 500 pg/mL) and ACTH (5 nM--positive control). All incubations were performed for 8 h (95% air and 5% CO2, 37 degrees C). ACTH increased cortisol and androstenedione secretion. Genistein, daidzein and biochanin A suppressed cortisol output, whereas androstenedione secretion was enhanced by these phytoestrogens. In contrast, oestradiol did not alter steroid secretion by porcine adrenocortical cells, which may suggest that phytoestrogens have a non-oestrogenic mechanism of action. Therefore, phytoestrogens present in commercial forage may influence adrenocortical function in pigs by decreasing cortisol and increasing androstenedione secretion.