The NF-E2-related factor 2 (Nrf2) transcription factor is a potent inducer of cytoprotective genes, which encode--among others--enzymes that detoxify reactive oxygen species (ROS). As we demonstrated a crucial role of Nrf2 in the prevention of skin carcinogenesis, it is of interest to identify Nrf2-activating factors in keratinocytes. For this purpose, keratinocytes from mice transgenic for an Nrf2-responsive reporter gene were analyzed. Electrophilic compounds activated the reporter in keratinocytes, and induced nuclear translocation of Nrf2 and the expression of known Nrf2 target genes. This is biologically relevant, as we show that Nrf2-mediated gene expression protects keratinocytes from the toxicity of these substances. By contrast, hydrogen peroxide, glucose oxidase, UVA, and UVB irradiation had no effect, although these treatments strongly increased the levels of intracellular ROS. To verify these results in vivo, transgenic reporter mice with and without functional Nrf2 alleles were topically treated with electrophilic chemicals or irradiated with UVB. Electrophiles but not UVB activated the reporter in an Nrf2-dependent manner. These results provide the basis for the identification of novel Nrf2 activators in keratinocytes with therapeutic potential for skin tumor prevention.