The subpopulation of myenteric neurons able to synthesize nitric oxide was studied quantitatively in the adult rat, using the NADPH-diaphorase histochemical method on whole-mount preparations of distended distal ileum. The spatial density of NADPH-diaphorase-positive myenteric neurons was 2388 +/- 193/cm2 (S.D.; five rats), comprising about 27% of the nerve cell bodies per ganglion. Most neurons were intensely stained and displayed predominantly a Dogiel type I morphology; about 8% of the labelled nerve cells were ovoid neurons, exhibiting a pale cytoplasmic reaction product. The mean somatic size of all NADPH-diaphorase-positive myenteric neurons was 446 +/- 40 microns2, with a mean nuclear size of 96 +/- 18 microns2 (mean values +/- S.D.; five rats). Such values fell exactly within the range of neuronal sizes of the total myenteric population, marked by means of NADH-diaphorase histochemistry. Therefore, the morphometric analysis did not identify any peculiar cell size feature, characterizing this specific nerve cell subpopulation. Thus, the present study provides quantitative data on the size, density and proportion of those myenteric neurons that may synthesize nitric oxide in the distal ileum of the rat.