Melatonin exhibits protective effects against ultraviolet radiation (UVR) via modulation of proinflammatory mediators and its free radical scavenging capacity. To date, several reports presented protective mechanisms of this agent against UVR-induced alterations in mitochondria and nuclei. This investigation evaluates the potent preventing action of melatonin regarding early-stage UVR-mediated perturbations in plasma membrane potential (mbΔΨ) and intracellular (cytosolic) pH (pHi) analyzed by flow cytometry. Experiments were carried out in a dose- and time-dependent manner using human keratinocytes [HaCaT and normal human epidermal keratinocytes (NHEK)]. First investigations, which used viability/cytotoxicity assays, showed the gradual mortality with increasing UVR doses and cultivation time. Pre-incubation with melatonin (10-3 M) prior to UVR exposure reduced lactate dehydrogenase release by 30% (HaCaT) and 28% (NHEK) at the dose of 50 mJ/cm 2 after 48 hr (P < 0.001). Furthermore, UVR caused hyperpolarization of mbΔΨ immediately (0 hr) after irradiation (25 or 50 mJ/cm2). At the dose of 50 mJ/cm2, cells cultivated for 48 hr manifested a marked increase in mbΔΨ by 112% (HaCaT) and 123% (NHEK). The presence of melatonin significantly protected the cells by 12% (HaCaT) and 14% (NHEK) (P < 0.001). Simultaneously, 50 mJ/cm2 induced dramatic acidification reaching after 24 hr the level of 6.40 (without melatonin), 6.56 (with melatonin) for HaCaT and 6.11 (without melatonin), 6.43 (with melatonin) for NHEK. The results presented provide information about the protective mechanisms of melatonin itself on one hand and, combined with data reported so far, confirm the potent antiapoptotic action of melatonin.