The membrane potential of vascular smooth muscle cells of the porcine coronary artery was measured to examine whether serotonin evokes endothelium-dependent hyperpolarization, and if it does, whether the electrical responses are modulated by the chronic dietary intake of NC 020, a defined fish oil. Serotonin induced transient, concentration-dependent hyperpolarizations of coronary arterial smooth muscle cells. The hyperpolarization was observed in tissues with, but not in those without endothelium. In coronary arteries obtained from pigs fed chronically with NC 020, serotonin induced significantly larger hyperpolarizations than those observed in control arteries. These results suggest that endothelium-dependent hyperpolarization may contribute to the endothelium-dependent relaxation evoked by serotonin and to its potentiation by the dietary intake of fish oil (NC 020).