Abstract: The β-receptor blocking action of methylflavonolamine hydrochloride(MFA) was studied and compared with those of propranolol. The doseresponse curves of isoproterenol were shifted to the right by MFA on isolated rabbit atrium in this experiment. The pA₂ value and the slope of the regression line of MFA calculated from Schild plot were 5.53 and -0.84 respectively. The effects of MFA and propranolol on duck erythrocyte membranes were studied by the radioligand binding method. Both MFA and propranolol inhibited the binding of ³H dihydroalprenolol to β-receptors. Their apparent equilibrium dissociation constants were 1.12×10^-5 mol/L and 5.50×10^-9 mol/L respectively. The affinity of propranolol to β-receptors of duck erythrocyte membranes was 2039-fold higher than that of MFA. These results demonstrates that MFA is a weak competitive β-receptor blocking agent.