In the myenteric plexus of the guinea-pig ileum, a sub-population of descending interneurons contains nitric oxide synthase. Final neurons in descending motility pathways, inhibitory circular muscle motor neurons, also contain nitric oxide synthase. In this study we used ultrastructural immunocytochemistry to determine whether nitric oxide synthase descending interneurons provide inputs to all nitric oxide synthase neurons. The presence of nitric oxide synthase inputs to 35 nitric oxide synthase nerve cells from three animals was examined. Nine nerve cells from one ganglion were studied in serial section. Every nerve cell received inputs (close contacts and synapses) from nitric oxide synthase terminals. The number of inputs to the nine serially sectioned neurons ranged from 13 to 45. The inputs were found in about equal numbers on the cell bodies and the dendrites. There was no significant correlation between the size of nitric oxide synthase neurons and the number of nitric oxide synthase inputs they received. There was also no correlation between the number of nitric oxide synthase inputs and the number of 5-hydroxytryptamine inputs (determined in a previous study) received by nitric oxide synthase neurons. Random sections through an additional 26 nitric oxide synthase neurons (seven in the same ganglion and 19 from another two myenteric ganglia from different animals) were examined and nitric oxide synthase synapses and close contacts were observed on each neuron. Nitric oxide synthase interneurons and motor neurons are morphologically indistinguishable. However, since all nitric oxide synthase neurons that were examined received inputs from nitric oxide synthase terminals, the nitric oxide synthase descending interneurons appear to provide inputs to both the nitric oxide synthase inhibitory motor neurons and descending interneurons. Hence the nitric oxide synthase descending interneurons are likely to play a direct role in descending motility reflexes, although nitric oxide does not appear to be the primary transmitter at neuro-neuronal synapses.