In vitrostudies on the new β-adrenoceptor antagonist, cetamolol (Betacor®), have demonstrated that the compound is a potent antagonist of the chronotropic effects of isoproterenol on guinea pig atria. The pA2value (8.05) of cetamolol was slightly lower than that of propranolol (8.44). The compound was shown to possess a moderate degree of cardioselectivity as indicated by a lower pA2value for the antagonism of isoproterenol-induced relaxation of the isolated guinea pig trachea (pA2 = 7.67) compared with that derived from atrial experiments (pA2 = 8.05). Up to concentrations of 10−4 M, cetamolol displayed negligible negative inotropic activity relative to propranolol in the electrically stimulated guinea pig left atrial preparation. When applied to isolated right atria from reserpinized rats, cetamolol had a positive chronotropic effect (approximately 75% of that displayed by practolol) which was antagonized by pretreatment with propranolol, thus indicating intrinsic sympathomimetic activity. Specificity experiments in a number of isolated tissues indicated that cetamolol had very little antihistaminic, anticholinergic, α1-adrenergic blocking, or calcium antagonistic properties. Biochemical receptor binding studies are in general agreement with the observations from the isolated tissue experiments.