Exposure to sources of UV radiation, such as sunlight, induces a number of cellular alterations primed by changes in the expression of a set of immediate early genes that include members of the AP-1 family of transcription factors. The Activating Protein 1 (AP-1) is a family of proteins composed by the Fos and Jan. sub-families. Several members of the Jim and Fos protein families play a central role in the transduction of environmental stimuli to the nucleus, as they are substrates for the Mitogen Activated Protein Kiaases (MAPKs). Stress Activated Protein Kisases (SAPKjs), are a subgroup of MAPKs, which include JNK kinase) and several p38 isoforms named p38ot, p38-l p38f, and p38o. Although activated bv receptor-linked signaling pathways, they constitute the main target for different sources of stress like changes the osniolarify of the media, inhibition of protein synthesis or UV radiation. In this regard, the phosphorylation of c-Jun by UV-induced JNK has been largely studied, whereas a role lor Fos proteins in UV-mediated responses and the identification of Fos activating kinases las remained conti’oversial. In this study, we have identified f>38 MAPKs as. proteins that can associate with Fos auct phosphorylate Its transactivatioti both in vitro said in vivo, This phosphorj-latioii is transduced, in changes in its transcriptional capability as p38-activated Fos induces AP1-dtiven gene expression. Moreover, our findings indicate that p38 MAPKs mediate c-Fos In response to UV light in parallel to the effect of JNK UV-induced c-jun phosphorylation and transcriptioiiai activation.