Abstract: Melatonin, which can be produced in the skin, exerts a protective effect against damage induced by ultraviolet radiation (UVR). The present study investigated the effect of UVB, the most damaging component of UVR, on melatonin metabolism in HaCaT keratinocytes and in a cell-free system. Four metabolites were identified by HPLC and LC-MS: 6-hydroxymelatonin, AFMK, 2-hydroxymelatonin (the main intermediate between melatonin and AFMK) and 4-hydroxymelatonin. Concentrations of these photoproducts were directly proportional to UVR-dose and to melatonin substrate content, and their accumulation was time dependent. The UVR-dependent increase of AFMK and 2-hydroxymelatonin was also detected in keratinocytes, where it was accompanied by simultaneous consumption of intracellular melatonin. Of note, melatonin and its two major metabolites, 2-hydroxymelatonin and AFMK, were also detected in untreated keratinocytes, neither irradiated nor preincubated with melatonin.