Background and purpose: We have previously shown that melatonin inhibits bradykinin-induced NO production by endothelial cells in vitro. The purpose of this investigation was to extend this observation to an in vivo condition and to explore the mechanism of action of melatonin. Experimental approach: RT-PCR assays were performed with rat cultured endothelial cells. The putative effect of melatonin upon arteriolar tone was investigated by intravital microscopy while NO production by endothelial cells in vitro was assayed by fluorimetry, and intracellular Ca 2 þ measurements were assayed by confocal microscopy. Key results: No expression of the mRNA for the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase, or for the melatonin MT 2 receptor was detected in microvascular endothelial cells. Melatonin fully inhibited L-NAME-sensitive bradykinin-induced vasodilation and also inhibited NO production induced by histamine, carbachol and 2-methylthio ATP, but did not inhibit NO production induced by ATP or a, b-methylene ATP. None of its inhibitory effects was prevented by the melatonin receptor antagonist, luzindole. In nominally Ca 2 þ -free solution, melatonin reduced intracellular Ca 2 þ mobilization induced by bradykinin (40%) and 2-methylthio ATP (62%) but not Ca 2 þ mobilization induced by ATP. Conclusions and implications:We have confirmed that melatonin inhibited NO production both in vivo and in vitro. In addition, the melatonin effect was selective for some G protein-coupled receptors and most probably reflects an inhibition of Ca 2 þ mobilization from intracellular stores.