The kinetic mechanism of super high affinity [3H]oxotremorine M binding to porcine m2 muscarinic receptors expressed in Chinese hamster ovary cells was examined. In cell lines expressing low receptor numbers (10(4) binding sites/cell) and in high expression (10(6) sites/cell) cell lines treated with cholate, [3H]oxotremorine M association and dissociation kinetics were monophasic. The reciprocal relaxation time for the association reaction was independent of [3H]oxotremorine M concentration and equaled the dissociation rate constant consistent with a special case for a mechanism involving a protein conformational change followed by ligand binding. Membranes from high expression cell lines and porcine atrial membranes showed complex kinetic behavior. Two kinetic phases were observed for [3H]oxotremorine M association, and both reciprocal relaxation times were independent of ligand concentration. The number of kinetic phases and their relative amplitudes seen in dissociation experiments were dependent on whether dissociation was initiated by dilution or by addition of unlabeled ligand(s) as well as on the fractional saturation of the receptor. Computer simulations of the data led to a model consistent with the existence of asymmetric receptor dimers as well as monomers and the ligand-dependent interconversion of fully occupied dimers and monomers.