Ligand interactions with porcine atrial muscarinic receptor solubilized in a mixed-detergent system (0.4% wt/vol digitonin and 0.08% wt/vol cholate) are described. The solubilized receptor interacts with ligands in a stereospecific manner, showing about the same affinity for local anesthetics and antagonists as was found for the membrane-bound protein. Agonists appear to interact with a single class of noninteracting sites that correspond to the low-affinity agonist sites in the membrane-bound preparation. Kinetic studies of L-[3H]quinuclidinyl benzilate binding to the receptor indicated a 2-step mechanism. The 1st step, in rapid preequilibrium (K = 5.7 .times. 10-9 M), was followed by a slow conformational change (k1 = 4 .times. 10-3 s-1; k-1 = 1.7 .times. 10-4 s-1) in the receptor-ligand complex. The overall dissociation constant calculated from the association kinetics (2.3 .times. 10-10 M) agreed well with the thermodynamic value for Kov (2.5 .times. 10-10 M); direct determination of k-1 gave a value .apprx. 4-fold lower(4.0 .times. 10-5 s-1) than predicted. Possible reasons for this discrepancy are discussed.