Uridine 5'-triphosphate (UTP) is stored in the granules of cells such as platelets and is released into the extracellular space upon cell stimulation. Extracellular UTP is known to influence many biological processes. We investigated the hemodynamic effects of UTP on the perfused rat liver and characterized its receptors. Liver perfusions were performed in a recirculation system under constant pressure (28 cmH₂O). The perfusion flow and oxygen consumption rate were measured at 30 second intervals. UTP decreased the perfusion flow and the oxygen consumption rate, dose-dependently. UTP-induced changes were transient and disappeared in about 10 minutes. Suramin (P₂-purinergic antagonist, 100 uM) and indomethacin (cyclooxygenase inhibitor, 20 uM) blocked UTP-induced hemodynamic changes significantly. The effects of UTP were also inhibited when Kupffer cells were damaged with treatment of gadolinium chloride (10 mg/kg iv). L-NAME (1 mM), a potent inhibitor of nitric oxide synthase, markedly enhanced and prolonged the contractile response of UTP in the hepatic vessel. These results suggest that UTP acts mainly on suramin-sensitive UTP receptors on the Kupffer cell through prostanoid synthesis. The nitric oxide systems in the endothelium seem to counteract the vasoconstrictile action of UTP in the hepatic circulation.