The enkephalins are two pentapeptides with pharmacological properties similar to narcotic drugs. These peptides are widely distributed in the central nervous system. The basal ganglia contain particularly high concentrations of enkaphalins. In the caudate nucleus ultrastructural studies show that enkephalin is contained within synaptic vesicles. Similar observations have been made with respect to enkephalin containing neurons in th A2 nucleus of the medulla and locus ceruleus. In the caudate, lower intensity staining for enkephalin can also be seen in the dendrites of enkephalin positive neurons. Staining for enkephalins is also widely distributed outside the central nervous system. The myenteric plexus in particular contains many enkephalinergic fibers and cell bodies. In addition, enkephalin-containing endocrine cells are found in the gastrointestinal mucosa. These endocrine cells also contain serotonin. Enkephalins have also been detected in several sympathetic ganglia. In some cases postganglionic neurons have been shown to contain enkephalin. SIF (small intensely fluorescent) cells and nerve terminals within sympathetic ganglia also show enkephalin positive staining. Many cells of the adrenal medulla show positive staining for enkephalin.