Noxious stimulation provokes the activation of genes that are thought to play a crucial role in the phenomena of stress and pain. Among these is the prodynorphin gene. By double-labeling in situ hybridization/immunohistochemistry, we show that increased prodynorphin gene expression is preceded, in the same neurons, by an early induction of c-fos. Inspection of the prodynorphin promoter region revealed the presence of several AP-1-like sequences. We demonstrate that only one of these sites is a functional AP-1 element. It is constituted by the noncanonical TGACAAACA sequence, in which the palindromic structure is partly conserved by the 3' terminal CA dinucleotide. Transfection experiments in NCB20 neuroblastoma cells indicated that this site is a target of Fos/Jun trans-activation. Our results suggest that Fos/Jun oncoproteins may function as third messengers in the signal transduction mechanisms of stress/pain processes.