The distribution and colocalization of nitrinergic and peptidergic nerves were examined in six human colons. The tissues were fixed, cryosectioned, and standard immunohistochemistry was performed for several known neuropeptides. The same sections were stained for NADPH-diaphorase to denote nitric oxide synthase. NADPH-diaphorase-positive myenteric neurons were counted and colocalization noted for each peptide, as well as for peptide terminations. Galanin was the only neuropeptide that colocalized to a significant extent (23.0 +/- 7.21%) with NADPH-diaphorase-positive myenteric neurons. Many neuropeptide-containing nerve fibers had extensive terminations onto NADPH-diaphorase-positive neurons. Vasoactive intestinal peptide was the only neuropeptide that colocalized with NADPH-diaphorase to any extent in nerve fibers within circular muscle (59.5 +/- 9.3%). Fiber distribution in the longitudinal muscles showed a similar, but less dense pattern. These observations provide morphological evidence for the presence of nitric oxide, a candidate nonadrenergic noncholinergic neurotransmitter in the human colon.