Pharmacological and biochemical evidence has implied that a widespread opioid peptide system exists within the cerebral cortex to mediate a variety of opiate effects. However, immunocytochemical detection of opioid peptides in the cortex has been limited. Using antisera to enkephalin and bovine adrenal medullary peptide, both fragments of proenkephalin, and an antiserum to dynorphin A, a fragment of prodynorphin, we now describe the regional and laminar distribution of a widespread population of olfactory cortical and neocortical cell bodies and fibers with opioid immunoreactivity in rats. Neurons stained with each antiserum are distributed bimodally in layers II and III and V and VI of neocortex as well as in layers II and III of olfactory cortex. The widespread distribution and heterogeneous morphology of cortical cells containing proenkephalin and dynorphin-A immunoreactivity suggest that opioid peptide-containing neurons may influence the functioning of local, commissural, and projection neurons in rat cerebral cortex.