This study was conducted to investigate the metabolism of arachidonic acid in the skin of hairless mice exposed to UVA, PUVA, UVB, and UVC irradiation. The main products of arachidonic acid in the epidermis were hydroxyeicosatetraenoic acid (HETE), PGE2, and PGD2. Dermis displayed a lower lipoxygenase activity (expressed as HETE production) than the epidermis and showed no detectable cyclooxygenase activity, i.e., no prostaglandin production. The main changes observed in UV-induced inflammatory reactions were as follows. 1. A 5-fold increase in dermal HETE production in PUVA-treated animals and a 29% reduction in epidermal HETE formation after UVC treatment. 2. A marked decrease of PGD2 and a marked increase of PGE2 formation due to alterations of PGH2 metabolism in the UVB-treated group; however, cyclooxygenase activity was unchanged. These changes in arachidonic acid metabolism in the skin may be of pathophysiologic importance in UV-induced inflammatory reaction.