Melatonin (Mel) is a hormone synthesized mainly by the pineal gland. The principal function of Mel in the body involves the control of circadian and seasonal rhythms. Moreover, numerous reports document its anti-oxidative properties. Skin and eyes are particularly sensitive to the noxious influences exerted by UV exposure. The most dangerous radiation of the UVB (ultraviolet-B) and UVA (ultraviolet-A) range induces the formation of reactive oxygen species and thus stimulates the apoptosis of exposed cells. In numerous in vivo and in vitro studies, Mel produced in the skin and eye has been found to protect against the sequelae of UVB- and UVA-induced oxidative stress. In in vitro studies involving UVB irradiation of keratinocytes, fibroblasts, and leukocytes, Mel applied in both pharmacological (10(-3) and 10(-4 )M) and physiological doses (10(-7) and 10(-9) M) decreased the fraction of damaged cells. A similar pattern of Mel action at various doses of Mel probably reflected the presence of melatonin receptors (mainly MT1 receptors) in skin and eye cells. Moreover, intraperitoneally administered Mel or Mel applied to the skin before UVB exposure protects against the development of cataract and erythema, respectively. Thus only intracellular Mel may protect cells against the effects of UVB exposure. Although there are numerous reports describing the effects of UVA on cells of the skin and eye, no studies have described the anti-oxidative properties of Mel in relation to UVA-irradiated cells.