The epidermal response to 2 different irritants, nonanoic acid (NAA) and sodium lauryl sulfate (SLS), was investigated with 2 different methods. NAA 80% and SLS 4% were applied under occlusion for up to 24 h. Elemental changes were determined in cryosections by x-ray microanalysis. Compared to unexposed skin a significantly higher sodium/potassium ratio was found after 6 h in NAA-exposed skin and a lower ratio in SLS-exposed. At 24 h both substances had induced similar changes, compatible with a cell injury. The findings demonstrate a time-dependent NAA and SLS response. With reverse transcription polymerase chain reaction, the mRNA expression of interleukin-1 alpha (IL-1 alpha), -1 beta (IL-1 beta), -6 (IL-6), and -8 (IL-8), tumor necrosis factor alpha (TNF alpha) and granulocyte macrophage colony stimulating factor (GM-CSF) in shave biopsies from irritated and unexposed skin was studied at 0. 4. 8 and 24 h. NAA, but not SLS, induced an increase in mRNA expression for IL-6 mRNA-expression for GM-CSF was increased after SLS exposure, but not after NAA. These findings indicate a time and substance dependent difference in the up-regulation of mRNA for different cytokines in epidermis during the first 24 h of the irritant reaction. This might be the effect of differences in the irritants action on the cell membranes, which is also reflected by the differences found in the elemental content at 6 h.