The effects of suramin on electrical and mechanical responses produced by adenosine triphosphate (ATP), acetylcholine and transmural nerve stimulation were observed in antrum smooth muscle isolated from the guinea-pig stomach. Suramin (>10(-6) M) inhibited the non-adrenergic non-cholinergic inhibitory junction potential, with no alteration of the resting membrane potential, slow wave and the ATP-induced responses (hyperpolarization and inhibition of slow waves). The amplitude, but not the frequency, of spontaneous rhythmic contraction was inhibited by suramin (>10(-5) M), with no alteration of electrical responses of the membrane. Transmural nerve stimulation elicited cholinergic excitatory and non-adrenergic non-cholinergic inhibitory responses on the spontaneous contraction, and suramin inhibited only the latter. Suramin did not alter the ATP-induced inhibition of spontaneous contraction. The contractions produced by low concentrations (<10(-7) M), but not high concentrations (10(-6) - 10(-5) M), of acetylcholine were inhibited by suramin. It is concluded that in smooth muscle of the guinea-pig antrum, suramin inhibits contractions produced spontaneously and by low concentrations of acetylcholine, with no relation to the electrical responses of the membrane. Parallel inhibition by suramin of the electrical and mechanical responses elicited by excitation of non-adrenergic non-cholinergic inhibitory nerves may not be causally related to the inhibition of ATP-receptors.