Nitric oxide (NO) is a recently discovered neurotransmitter that is thought to mediate relaxation of gut smooth muscle during peristalsis. To assess its role in the pathophysiology of Hirschsprung's disease, the authors examined the distribution of neurons that produce NO in specimens from seven infants with this condition. Immunohistochemical analysis of cryostat sections for nitric oxide synthase (NOS) immunoreactivity (NOS catalyzes the production of NO) showed that NOS is localized in a substantial subpopulation of enteric neurons in both the myenteric and submucosal plexuses in the ganglionated gut, but it was completely absent in aganglionic bowel. NOS immunoreactivity specifically colocalizes in neurons that also contain NADPH-diaphorase activity. This finding enabled the distribution of NO-producing neurons to be determined using whole-mount histochemistry, a technique that allows the enteric neural network to be examined intact. In normal bowel, NO-producing neurons are arranged in star-shaped myenteric and submucosal ganglia, which are joined to one another by nerve fibers to form a meshwork of variable geometry. Individual neurons exhibit Dogiel type 1 morphology. Axonal processes leave the myenteric plexuses and lie parallel to muscle bundles in both muscle layers of the gut. In the transition zone, ganglia are initially present, but their orientation changes so that both they and the internodal strands that connect them are aligned linearly along the craniocaudal axis of the gut tube. More distal still, ganglia and then all NOS activity disappear completely. These results suggest that in Hirschsprung's disease, the failure of aganglionic bowel to relax during peristalsis might be caused by the absence of NO-producing neurons.