The effect of melatonin on the Ca(2+) signaling process in bovine aortic endothelial cells (BAE) and in primary cultured vascular endothelial cells from normotensive Sprague Dawley (SDR) and genetically hypertensive (SHR) rats was investigated using the Ca(2+) indicator Fura-2. Acute applications of melatonin failed to initiate a Ca(2+) response in the three cell types considered. However, preincubating SHR aortic endothelial cells with exposure to melatonin increased the internal Ca(2+) release triggered by bradykinin (BK) and ATP while stimulating the related agonist-evoked Ca(2+) entry. This effect appeared specific for SHR cells, as a similar incubation period failed to alter the Ca(2+) responses in BAE and SDR cells. Because of the known overproduction of free radicals in SHR cells, the effect of melatonin on Ca(2+) signaling was also tested in SDR and BAE cells exposed to the superoxide anion radical. Melatonin reversed the deleterious action of free radicals on Ca(2+) signaling in both cases, suggesting that its stimulatory effect in SHR was linked to its antioxidative properties. Finally, experiments where melatonin was applied between successive BK stimulation periods showed an enhancement of the agonist-evoked Ca(2+) entry in BAE and SDR cells. This effect appeared to be independent of the production of second messengers as no specific binding sites for melatonin, including MT1, MT2 and MT3 receptors, could be detected in BAE cells. We conclude that melatonin improves Ca(2+) signaling in dysfunctional endothelial cells characterized by an overproduction of free radicals while stimulating the agonist-evoked Ca(2+) entry in normal endothelial cells through a mechanism not related to its antioxidative properties.