Vasopressin, a vasoactive peptide, causes vasoconstriction via V1a vasopressin receptors. Unlike other vasoconstrictor agents, vasopressin also has vasodilatory properties. The purpose of this study was to determine the effect of vasopressin on hepatic and splanchnic circulation in SpragueDawley rats. The experiments were conducted in not only isolated blood-and constant flowperfused livers but also anesthetized spontaneously breathing rats. In anesthetized rats, portal venous pressure (Ppv), systemic arterial pressure (Psa), central venous pressure, and hepatic blood flow (HBF) of combined portal venous and hepatic arterial blood flow were continuously measured, and splanchnic vascular bed resistance (Rspl) defined by (Psa − Ppv) / HBF was determined. In perfused livers, vasopressin at 0.1-1,000 nM caused weak venoconstriction as evidenced by small increase in Ppv. In anesthetized rats, when vasopressin was injected into the portal vein as a bolus consecutively at 0.01-100 nmol/kg, Psa increased dose-dependently with the peak increment of 60 ± 18 mmHg at 100 nmol/kg. Ppv and HBF decreased, with resultant increase in Rspl, indicating splanchnic vasoconstriction. In conclusion, hepatic venoconstrictor action of vasopressin was weak in rats. Vasopressin causes splanchnic vasoconstriction, resulting in a decrease in HBF and Ppv in anesthetized rats.Vasopressin, a vasoactive peptide, causes vasoconstriction via V1a vasopressin receptor of vascular smooth muscle cells (1, 5). Vasopressin has been a well-established therapeutic agent controlling severe hypotension such as hemorrhagic shock due to gastrointestinal bleeding (21), septic shock (6, 8), and anaphylactic shock (13). Its beneficial effects on hypotension are usually ascribed to its strong arteriolar constrictor activity in most peripheral vascular beds, especially splanchnic vascular beds. Vasopressin-induced vasoconstriction in the intestinal and splenic vascular beds results in a reduction of portal blood flow with a fall in hepatic portal venous pressure (Ppv). Vasopressin causes vasoconstriction in most vascular beds. Paradoxically, vasopressin has also been demonstrated to cause vasodilation in numerous vascular beds (2,3,9,10,12,16,17,19), a feature not shared by other vasoconstrictor agents. For rats which are one of the most frequently used animals for experiments, the investigations on the effect of vasopressin on the splanchnic vascular bed in vivo are limited (11): blood flow to liver was not measured, although a decrease in Ppv was reported. A decrease in Ppv could be theoretically induced by vasodilation, rather than vasoconstriction, in the hepatic vessels. On the other hand, there are reports on the hepatic vascular response of perfused rat livers to vasopressin, indicating absence of constriction or dilatation (14,18). However, these experiments