Ozone (O 3 ) is the major air pollutant in photochemical smog. Since O 3 is a highly reactive oxidant, we hypothesized that O 3 induces oxidative stress in the outermost epidermal layers. SKH-1 hairless mice were exposed for 2 h to 10 ppm O 3 (n=4) or to 0 ppm O 3 (controls, n=4). Subsequently, antioxidants (vitamins C and E) and malondialdehyde (MDA, a parameter of lipid peroxidation) in 3 different layers of skin (upper epidermis, lower epidermis/papillary dermis, and dermis) were measured using HPLC techniques. In the upper epidermis, 10 ppm O 3 depleted vitamin C by 55% (p<0.01), vitamin E by 22% (p<0.05) and increased MDA levels 10-fold (p<0.001). In the lower epidermis/papillary dermis, MDA was increased 2-fold (p<0.05), while levels of vitamins C and E remained unchanged; dermal skin did not contain any evidence of oxidative stress. Based on these findings, we hypothesized further that the stratum corneum (SC), as the outermost layer of skin and first line of defense against environmental attack, is particularly sensitive to O 3 . To evaluate this, SKH-1 hairless mice were exposed for 2 h to single doses of 0 ppm O 3 (n=12), 1, 5, and 10 ppm (each level n=4) or repeated 2 h exposures on 6 consecutive days to 0 ppm (n=4) or 1 ppm (n=4). New HPLC methods were developed to measure vitamin E and MDA in tape strippings of SC. O 3 depleted SC vitamin E and caused MDA formation in the SC in a dose dependent manner. Remarkably, repeated O 3 -exposures to 1 ppm significantly increased oxidative damage as measured by vitamin E depletion (p<0.001) and MDA formation (p<0.01), indicating cumulative injury at low O 3 -doses. This is the first demonstration of oxidative injury in the upper epidermis and the SC by O 3 . Our results further suggest that chronic exposures to O 3 may be involved in triggering skin pathologies that are encountered with increasing prevalence in urban smog areas.