Antisera raised against synthetic dynorphin or [Leu5]enkephalin demonstrate immunostaining in hippocampal mossy fibers and in dentate granule cells. However, dynorphin immunoreactivity (ir) appears to be denser in immunocytochemical preparations and is quantitatively greater by radioimmunoassay than enkephalin-ir. Immunostaining with dynorphin antisera is eliminated by adsorption with 1-100 microM dynorphin-17 whereas immunostaining with enkephalin antisera is eliminated by adsorption with 1-100 microM [Leu5]enkephalin, dynorphin-17, dynorphin-(1-13), or alpha-neo-endorphin. Intrahippocampal colchicine injections, which selectively destroy dentate granule cells, significantly decrease the dynorphin-ir and enkephalin-ir levels in rat hippocampus. Intraventricularly administered kainic acid, which selectively destroys CA3-4 pyramidal cells, results in an increase of enkephalin immunostaining in mossy fibers and a significant increase in enkephalin-ir by radioimmunoassay in whole hippocampus. The enkephalin-ir cells and fibers in entorhinal/perirhinal cortex, which innervate rat hippocampus and dentate gyrus, do not contain dynorphin-ir.