We studied the interaction of disopyramide, quinidine, and procainamide with cardiac muscarinic receptors. In electrophysiological experiments, the effects of disopyramide, quinidine, procainamide, and atropine were determined on spontaneously depolarizing guinea pig right atria (GPRA) both in the presence and absence of pharmacologically induced (physostigmine) cholinergic stimulation. All four agents demonstrated a concentration-dependent antagonism of the negative chronotropic effects of physostigmine. The order of anticholinergic potency was atropine » disopyr-amide > quinidine » procainamide. The ability of disopyramide to antagonize the physostigmine induced slowing was stereoselective, (+)disopyramide > (—)disopyramide. In contrast, the ability of quinidine to antagonize the negative chronotropic effects of physostigmine was non-stereoselective, quinidine = quinine. In parallel experiments, we studied the ability of disopyramide, quinidine, procainamide, and atropine to compete with the radiolabeled muscarinic receptor antagonist [3H] quinuclidinyl benzilate ([3H]QNB) for binding to muscarinic receptors in crude homogenates of GPRA and membrane vesicles from canine ventricular myocardium. All four agents inhibited [3H]QNB binding to muscarinic receptors. The order of anticholinergic potency determined by the receptor binding studies was identical to that determined by the physiological studies. The interaction of disopyramide with muscarinic receptors was stereoselective, (+)disopyramide > (−)disopyramide. Quinidine was only slightly more potent than quinine in inhibiting [3H]QNB binding to muscarinic receptors. Inter-action of antiarrhythmic drugs with muscarinic receptors satisfied criteria for a competitive interaction. The data from this study localize the anticholinergic effects of disopyramide and quinidine to the muscarinic receptor.Circ Res 47: 855-865, 1980