To investigate the mechanisms of splanchnic hyperdynamics in portal hypertension (PHT), angiotensin Ⅱ(A-Ⅱ) receptor maximal binding capacity (Bmax) and dissociation constants (Kd) of splanchnic blood vessels in rats with prehepatic PHT were studied by radioligand binding analysis. The results showed that the A-Ⅱ receptor Bmax in the superior mesenteric artery and portal vein of PHT animals (206.9±39.3 fmol/mg protein and 31.5±9.2 fmol/mg protein respectively) was all significantly lower than that of the controls (297.2±44.7 fmol/mg protein and 53.4±12.1 fmol/mg protein respectively, Ｐ＜0.01). The A-Ⅱ receptor Kd in the superior mesenteric artery was markedly increased in PHT animals (1.03±0.11 nmol/L) compared with that in controls (0.88±0.08 nmol/L, Ｐ＜0.05). In the portal vein, the A-Ⅱ receptor Kd in PHT animals was slightly higher than in controls, but no significant difference was observed between the two groups. These results suggest that the vascular hyporesponsiveness to A-Ⅱ in PHT is caused partially by a reduction in number and a decrease in affinity of vascular A-Ⅱ receptors, and these changes may possibly lead to the formation of hyperdynamic circulation.