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Melatonin is an important natural oncostatic agent. At present there are no data available as to its possible influence on tumor angiogenesis, which is a major biological mechanism responsible for tumor growth and dissemination. It is well known that vascular endothelial growth factor (VEGF) is crucial to a solid tumor's higher vascularization and development. To investigate the possible influence of melatonin on angiogenesis, we studied the effect of melatonin on endogenous VEGF expression in three human cancer cell lines (PANC-1, HeLa and A549 cells). In this study, we report that physiologic concentrations of melatonin have no obvious impact on the VEGF expression, whereas pharmacologic concentrations of melatonin suppress the VEGF mRNA and protein levels induced by hypoxia mimetic cobalt chloride (CoCl2). Melatonin also decreases hypoxia-inducible factor (HIF)-1α protein levels, suggesting a role for transcription factor HIF-1 in the suppression of VEGF expression. The effect of pharmacologic concentrations of melatonin on VEGF and HIF-1α under normoxia is uncertain, which indicates that the regulatory mechanisms of VEGF in the absence or presence of CoCl2 are different and other or additional transcription factors may be involved. Taken together, our data show that melatonin in high concentrations markedly reduces the expression of endogenous VEGF and HIF-1α induced by CoCl2 in cultured cancer cells.