A protein capable of binding atropine and (3H)propylbenzilylcholine mustard was solubilized and purified (200-fold) from rat brain. Pronase and trypsin, but not phospholipases, diminished the binding capacity of the solubilized receptor. The molecular weight of the salt-solubilized receptor as determined by gel filtration in the absence of detergents is 30,000. The purified protein showed specificity of binding toward muscarinic ligands. the high and low affinity dissociation constants of the receptor.atropine complex are 0.3 nM and 0.15 muM. Binding of atropine is pH-dependent with an optimum at 7.1. Ca2+ influences the binding of atropine and maximal binding occurs at 0.5 mM Ca2+. The subcellular distribution of the receptor was also examined.