Functional muscarinic receptors mediating guinea-pig gallbladder contractions were reported to be of M 4 subtype depending on the antagonistic affinities of several selective antagonists. Binding data indicate that both M 2 and M 4 receptors are present in this tissue and most antagonists are unable to discriminate between M 2 and M 4 sites. McKinney et al. have reported that oxotremorine tended to bind more potently to M 2 receptors than McN-A 343 but the two agonists were nearly equally potent at M 4 subtypes. In the present study, the agonistic affinities of carbachol (CCh), oxotremorine (Oxo) and McN-A 343 were investigated at isolated guinea-pig gallbladder and electrically stimulated rat left atrium. The cumulative concentration-response curves to CCh, Oxo and McN-A 343 were obtained and EC 5 0 values were calculated. The agonists caused contractile responses at gallbladder and negative inotropic effects at atrial strips.The rank order of the potencies of the three agonists were as follows: atrium, Oxo > CCh > McN-A 343; gallbladder, McN-A 343 > Oxo > CCh. It may be concluded that the functional muscarinic receptors at rat atrium and guinea-pig gallbladder are not of the same subtype, but M 2 and M 4 , respectively.