In circular smooth muscle bundles isolated from the guinea-pig stomach antrum, the effects of quinidine, Ni²⁺, flufenamic acid, niflumic acid, La³⁺, SKF-96365 and 4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) on acetylcholine (ACh)-induced depolarization were investigated. Recording membrane potentials from smooth muscle cells with intracellular microelectrodes revealed that ACh (1 μM) depolarized the membrane by 5-8 mV and increased the amplitude and frequency of slow potentials. These effects were inhibited by atropine. Quinidine (10 μM) increased the amplitude of ACh-induced depolarization, with no alteration to the properties of slow potentials. Ni²⁺ (50 μM) transiently (5-10 min) depolarized the membrane by about 5 mV, with an associated increase in frequency and amplitude of slow potentials. In the stabilized condition with Ni²⁺, the amplitude of ACh-induced depolarization remained unchanged. Flufenamic acid (10 μM) inhibited the generation of slow potentials, with no change in either the amplitude of ACh-induced depolarization or of the amplitude and frequency of slow potentials generated during ACh stimulation. A high concentration of flufenamic acid (100 μM) depolarized the membrane and increased the amplitude of ACh-induced depolarization. Niflumic acid (10 μM) hyperpolarized the membrane and increased the amplitude and frequency of slow potentials and also the amplitude of ACh-induced depolarization. DIDS (100 μM) hyperpolarized the membrane and inhibited the amplitude and frequency of slow potentials, with no alteration to the amplitude of ACh-induced depolarization. SKF-96365 (3-50 μM) depolarized the membrane in a concentration-dependent manner, but did not change the level of ACh-induced depolarization. La³⁺ (50 μM) did not alter the properties of the slow potentials or the ACh-induced responses. These results provide evidence that ACh-induced depolarization is not inhibited by chemicals known to inhibit non-selective cation channels. We suggest that muscarinic receptor-mediated signal transduction may be different in smooth muscle and interstitial cells.