The antimuscarinic properties of pirmenol(Pr) and penticainide(Pt), new class Ia antiarrhythmic agents, were compared with those of disopyramide(D) in guinea pig left atrium(LA), urinary bladder(Bl) and submandibular gland(Gl). Pr, Pt and D all caused concentration-dependent rightward shift of the concentration-response curves for the carbachol-induced negative inotropic effect in LA and contraction in Bl. The antimuscarinic potencies of D and Pt in LA were similar to those in Bl. However, Pr was ten times more potent in LA than in Bl. The orders of antimuscarinic potency were Pr>D>Pt in LA and D>Pr>Pt in Bl. All the agents inhibited specific ^^3 H-N-methylscopolamine(NMS) binding to membranes from LA, Bl and Gl. Computer-assisted analysis of the displacement curves by Pr showed that Pr binds to one site in both LA and Gl. The Ki value for Pr in Gl was ten times greater than that in LA. In Bl, Pr was found to bind to two distinct affinity sites. The K_H and K_L values in Bl were comparable to the Ki values obtained in LA and Gl, respectively. On the other hand, both D and Pt inhibited ^^3 HNMS binding in LA in a manner analogous to that observed in Bl. In conclusion, while D and Pt does not exhibit selective affinity for muscarinic receptor subtypes, Pr may discriminate between cardiac and glandular subtypes of muscarinic receptors.