The distribution of dynorphin-like immunoreactivity in the autonomic nervous system of the rat and guinea-pig was investigated using an antiserum raised against dynorphin-(1-17). Dynorphin-like immunoreactivity was observed in fiber networks in the prevertebral sympathetic ganglia, in fibers in the enteric plexa , circular muscle layer and a few in the lamina muscularis of the mucosa of the gastrointestinal tract. After colchicine treatment dynorphin-immunoreactive cell bodies were observed both in the myenteric and submucous ganglia of the gut. The paravertebral ganglia contained occasional dynorphin-positive fibers and a few immunoreactive small intensely fluorescent cells. The distribution of the dynorphin-like immunoreactivity was compared to the distribution of enkephalin-like immunoreactivity, and it was found that in some areas there were similarities in the distribution patterns, although in other areas there were clear differences, indicating a non-identity of these two systems.