Tyrosine kinases have been implicated in vascular smooth muscle cell proliferation and contraction. The involvement of tyrosine kinases in 5-hydroxytryptamine (5-HT)-induced contractile response of the isolated aorta was examined in two-kidney, one clip (2K1C) hypertensive rats, 2141C hypertension was made by constricting the left renal artery and age-matched control rat received sham treatment. Thoracic aortic rings denuded of endotheliurn were mounted in tissue bath, for measurement of isometric contractile force. The putative tyrosine kinase inhibitor, genistein, shifted concentration-response curves to 5-HT toward the right in control rats The isometric force generation induced by 5-HT was also inhibited by genistein in aortic rings from 2K1C: hypertensivc rats, however genistein did not affect on the high concentration of 5-HT in hypertensive rat ;. Genistein-induced relaxations were more attenuated in aortae from hypertensive rats than those from control Genistein had no effect on contrartcion elicited by the direct protein kinase C activator, phorbol 12, 13 dibutyrate (PDBu) either in 2KlC hypertensive or in control Group. These findings indicate that genistein-sensitive tyrosine kinases paeticipate in 5-HT-induced contraction of rat aortic smooth muscle, of which role is apparent in 2K1C: hypertension.