Substance P (SP) and bradykinin (BK) relax and hyperpolarize smooth muscles of pig coronary arteries in an endothelium-dependent manner. We investigated the effect of both peptides on the membrane potential of endothelial cells in culture. The membrane potential of pig coronary artery endothelial cells in primary culture was -44 mV. SP and BK hyperpolarized it transiently by 23 mV. These hyperpolarizations were dependent on the potassium gradient across the cellular membrane. They had similar time courses as the SP and BK endothelium-dependent hyperpolarizations already observed in smooth muscles.