The binding properties of oxomemazine to muscarinic receptors using the ability of oxomemazine to inhibit [³H]QNB binding in membrane fractions of rat cerebrum and guinea pig ventricle and ileum were investigated. [³H]QNB bound to a single class of muscarinic receptors with a dissociation constant of approximately 60 pM in three tissue preparations. Pirenzepine and oxomemazine inhibited [³H]QNB binding in cerebrum with a Hill coefficient lower than unity, and the inhibition data were best described by a two-site model. The relative densities of the high (M₁) and low (M₂) affinity sites for pirenzepine were 60 and 40%, with corresponding Ki values of 16 and 431 nM, and those (O_H and O_L) for oxomemazine 40 and 60%, with corresponding Ki values of 80 and 1350 nM. However, the inhibition data of both drugs vs [³H]QNB in ventricle and ileum appeared to obey the law of mass-action (Hill coefficient close to 1). The apparent Ki values of pirenzepine were 850 and 250 nM, and those of oxomemazine 1460 and 570 nM in ventricle and ileum, respectively. Thus, oxomemazine like pirenzepine has high affinity for cerebrum, moderate affinity for ileum and low affinity for ventricle. These results suggest that oxomemazine could recognize the muscarinic receptor subtypes with a high affinity for the M₁ sites.