We recently demonstrated the occurrence and functional roles of plasma membrane estrogen receptors in cultured bovine adrenal medullary cells (Biochem Biophys Res Commun., 339: 548–553, 2006). Here we report the effects of daidzein, a phytoestrogen of soy beans, on catecholamine synthesis and secretion in the cells. Incubation of cells with daidzein for 20 min increased the synthesis of ¹⁴C-catecholamines from [¹⁴C]tyrosine, but not from [¹⁴C]DOPA, in a concentration-dependent manner (10 – 1000 nM). The stimulatory effect of daidzein on ¹⁴C-catecholamine synthesis was not inhibited by ICI182,780, a classical estrogen receptor inhibitor. Acetylcholine, a physiological secretagogue, stimulated the synthesis of ¹⁴C-catecholamines, which was suppressed by daidzein at 1 µM. Daidzein at high concentrations (1 – 100 µM) suppressed catecholamine secretion induced by acetylcholine. Furthermore, daidzein (10 – 1000 nM) inhibited the specific binding of [³H]17-estradiol to plasma membranes isolated from bovine adrenal medulla. The present findings suggest that daidzein at low concentrations stimulates catecholamine synthesis through plasma membrane estrogen receptors but at high concentrations inhibits catecholamine synthesis and secretion induced by acetylcholine in bovine adrenal medulla. The latter effect of daidzein may be a beneficial action on the cardiovascular system.