BACKGROUND AND PURPOSE: TRPC5 is a Ca(2+)-permeable channel with multiple modes of activation. We have explored the effects of genistein, a plant-derived isoflavone, on TRPC5 activity, and the mechanism(s) involved. EXPERIMENTAL APPROACH: Effects of genistein on TRPC5 channels were investigated in TRPC5-over-expressing human embryonic kidney 293 (HEK) cells and bovine aortic endothelial cells (BAECs) using fluorescent Ca(2+) imaging and electrophysiological techniques. KEY RESULTS: In TRPC5-over-expressing HEK cells, genistein stimulated TRPC5-mediated Ca(2+) influx, concentration dependently (EC(50)= 93 microM). Genistein and lanthanum activated TRPC5 channels synergistically. Effects of genistein on TRPC5 channels were mimicked by daidzein (100 microM), a genistein analogue inactive as a tyrosine kinase inhibitor, but not by known tyrosine kinase inhibitors herbimycin (2 microM), PP2 (20 microM) and lavendustin A (10 microM). Action of genistein on TRPC5 channels was not affected by an oestrogen receptor inhibitor ICI-182780 (50 microM) or a phospholipase C inhibitor U73122 (10 microM), suggesting genistein did not act through oestrogen receptors or phospholipase C. In BAECs, genistein (100 microM) stimulated TRPC5-mediated Ca(2+) influx. In patch clamp studies, both genistein (50 microM) and daidzein (50 microM) augmented TRPC5-mediated whole-cell cation current in TRPC5 over-expressing HEK cells. Genistein stimulated TRPC5 channel activity in excised inside-out membrane patch, suggesting that its action was relatively direct and did not require cytosolic factors. CONCLUSIONS AND IMPLICATIONS: The present study is the first to demonstrate stimulation of a TRP channel by isoflavones. Genistein is a lipophilic compound able to stimulate TRPC5 activity in TRPC5-over-expressing HEK cells and in native vascular endothelial cells.