1. The effects of chebulinic acid, which has been shown to elicit blood pressure lowering effect in rats, on aortic vascular contraction as well as cardiac contraction were studied in rats. 2. Chebulinic acid had no effect on KCI-induced aortic contraction, but irreversibly inhibited the contractile responses to phenylephrine in an apparently non-competitive manner. Chebulinic acid also inhibited contractile responses of rat aorta to 5-hydroxytryptamine and angiot ensin It. 3. Chebulinic acid inhibited the binding of pH]-prazosin to dog aortic microsomal membranes in a concentration-depen-dent manner with an IC50 value of 0.3dmmol/L Results of saturation binding experiments suggest a mixed mode of inhibition by chebulinic acid (i.e. a decrease in both the maximal number of binding sites and the affinity for prazosin). 4. Chebulinic acid concentration-dependently and reversihly inhibited the maximal left ventricuiar pressure of rat heart in a Langendorff preparation with 50% inhibition occurring at a concentration of 0.3nmol/L. 5. We conclude that chebulinic acid exerts non-specific inhibitory actions in vascular preparations. Its inhibitory effect on cardiac contraction was reversible and three orders of magnitude more potent than that on vascular contraction. We suggest that the hypotensive effect of chehulinic acid is probably mediated via the decrease in cardiac output resulting from reduced left ventricular contraction.