We explored the effect of inhibitor of mitochondrial Na(+)-Ca(2+)-exchanger CGP37157 and the effect of mitochondria permeability transition pore inhibitor, cyclosporine A, on the membrane potential and electrical responses to endothelium-dependent dilators in intact endothelial cells from excised rat aorta and EA.hy 926 endothelial cells. Cyclosporin A did not affect the resting membrane potential and hyperpolarization to acetylcholine and histamine in intact and cultured cells. In contrast, CGP37157 (20 mcM) evoked membrane depolarization in unstimulated cells and suppressed the sustained component of hyperpolarization to acetylcholine and histamine both in intact and cultured endothelial cells, respectively. This was accompanied by a pronounced depolarization with membrane potential oscillations, which were not observed in the absence of Ca2+. We conclude that CGP37157 modulates endothelial membrane potential at rest and electrical responses to endothelium-dependent dilators. Possible mechanisms accompanying the CGP37157 action are discussed.