An in situ hybridization study showed that limbic seizures induced by kainate strongly augmented the prodynorphin and proenkephalin messenger RNA levels in granular cells of the rat hippocampal dentate gyrus. Pentylenetetrazole increased the level of proenkephalin messenger RNA, but slightly decreased that of prodynorphin messenger RNA in the dentate gyrus. Administration of kainate to rats caused a profound increase in messenger RNAs of the transcription factor genes c-fos and c-jun in the dentate gyrus, followed by an increase in the level of the transcriptional complex activator protein-1 in hippocampal neurons. Pentylenetetrazole also elevated the formation of activator protein-1, but the effect appeared earlier than that induced by kainate. Thus, recurrent limbic seizures activate both prodynorphin and proenkephalin genes, whereas generalized clonic-tonic seizures seem to decrease the prodynorphin and increase the proenkephalin gene expression in the dentate gyrus. Furthermore, our present results suggest that the transcription factors, c-fos, c-jun and activator protein-1 complex may be involved in the process of inducing the hippocampal proenkephalin gene, while these factors might be differently involved in regulation of prodynorphin gene expression.