The effects of eperisone, an antispasmodic, on the electrical and mechanical properties of smooth muscle cells of the guinea-pig basilar artery were investigated. Eperisone (below 50 microM) did not modify the membrane potential, membrane resistance, excitatory junction potential evoked by perivascular nerve stimulation or the K- and 5-hydroxytryptamine-induced depolarizations. However, in concentrations over 10 microM, this agent reversibly blocked the spontaneously generated and evoked action potentials in the presence of 5 to 8 mM tetraethylammonium. The contractions evoked by high concentrations of K, 5-hydroxytryptamine or direct muscle stimulation in the presence of tetraethylammonium were dose dependently inhibited by eperisone, in concentrations over 1 microM. In Ca-free 2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid-containing solution, 5-hydroxytryptamine produced only a single contraction. Eperisone inhibited this contraction to the same extent as that observed in the presence of Ca. These results indicate that eperisone possesses the property of a Ca antagonist on smooth muscle tissues of the guinea-pig basilar artery, in addition to the action of antispastic agent, i.e., this agent blocks the voltage-dependent influx of Ca at the smooth muscle membrane, but not at the nerve terminals, and inhibits the action of Ca in cells through inhibition of the contractile protein.