Mefloquine, an antimalarial drug, has been reported to block exogenously transfected gap junctions composed of either C x 36 or C x 50 more potently than those composed of other connexins. Using the conventional whole-cell clamp technique, we investigated the effects of mefloquine on intercellular electrical coupling in vascular endothelial cells of guinea-pig mesenteric arteries, where expressions of C x 40 and C x 43 have been identified. Mefloquine (50 microM) almost abolished the current required to impose a 10 mV command step, leaving only rapid capacitive currents and some sustained currents after about 3 min. The relaxation of the capacitive current could be well fitted with a single exponential function. The effect of mefloquine was reversible and the time course of the current induced by the voltage step gradually changed back after mefloquine was removed. The mean input resistance and capacitance in the presence of mefloquine were 323 MOmega and 10.1 pF, respectively. While intercellular electrical coupling was well blocked by mefloquine (50 microM), the membrane hyperpolarized from -24.0 to -32.5 mV. The results indicate that mefloquine effectively blocks gap junctions without producing major side effects in vascular endothelial cells and that this compound is a useful tool in the investigation of gap junctions.