A β-adrenoceptor blocking agent, Befunolol, is found to has intrinsic sympathomimetic activities in isolated right atria, trachea and taenia caecum of guinea pig (intrinsic activities are 0.22-0.28). The pD2-values of Befunolol estimated in the isolated organs are significantly different from its pA2-values against isoprenaline. Befunolol interacts with the beta-adrenoceptor where there may be two different sites: one site for agonistic action and the other for competitive antagonistic action. The intrinsic activity of Befunolol may be equal to its selectivity for both the sites[1].