The cyclic nucleotides such as ATP and UTP are potential humoral regulators of vascular tone. ATP has been shown to be released from sympathetic neurons and plasma levels of ATP have been shown to be relatively high. The present study was undertaken to investigate the mechanism(s) which mediate vasodilator responses to ATP and UTP in the hindlimb vascular bed of the cat under constant-flow conditions. Intraarterial injection of ATPγS, ATP and UTP in doses of 0.03-100 μg induced dose-related decreases in hindlimb perfusion pressure. The order of potency was ATPγS > ATP > UTP. The vasodilator responses to ATP and UTP were not antagonized by administration of the nitric oxide synthase inhibitor, L-NAME, the K+ATP channel inhibitor, U37883A, or the the cyclooxygenase inhibitor, meclofenamate. Neither atropine, nor propranolol had significant effects on vasodilator responses to ATP or UTP. The duration of the vasodilator response to ATP and UTP was increased after administration of the cAMP selective phosphodiesterase inhibitor, rolipram, while the cGMP phosphodiesterase inhibitor, Zaprinast was without effect. Results from the present study provide evidence that ATP and UTP have potent vasodilator activity in the hindlimb vascular bed of the cat and that this activity is mediated, in part by an increase in smooth muscle cAMP levels.