alpha,beta-Methylene ATP (alpha, beta-mATP), ATP and UTP dose dependently increased the perfusion pressure of rat mesenteric arteries with a potency order of alpha, beta-mATP >> ATP > UTP. In the veins, while alpha, beta-mATP did not affect the pressure, both ATP and UTP equi-potently increased it. The arterial ATP response was attenuated to some degree by suramin (100 microM), but markedly and to a similar extent by pyridoxal-phosphate-6-azophenyl-2',4-disulphonic acid (PPADS 30 microM) and alpha, beta-mATP (100 nmol). The venous response was not affected by PPADS or alpha, beta-mATP, but was slightly attenuated by suramin. Thus, ATP seems to elicit arterial constriction predominantly by stimulating P2X, but venous constriction by stimulating P2U purinoceptors.