Parasympathetic innervation of the heart, mediated in the atria by the muscarinic acetylcholine receptor; is known to be involved in the genesis and facilitation of a number of cardiac arrhythmias. In order to gain insight in to the molecular mechanism and composition of the muscarinic receptor, the purification of this protein is proposed using techniques such as affinity chromatography. Hydrodynamic studies are proposed in order to gain information on the apparent molecular weight and shape of the solubilized protein. Thermodynamic studies of the effects of putative regulatory agents such as guanine nucleotides are proposed in order to determine the mechanism by which these compounds affect receptor interactions with muscarinic agonists and antagonists. Information obtained as a result of these studies will increase our level of understanding of neural control of the heart and may lead to the development of more effective antiarrhythmic agents.