The nitric oxide producing neurons of the human colonic myenteric plexus have been studied by using antibodies against cerebellar NO synthase type I (NOS-IR) and NADPH-diaphorase (NAPDH-d) histoenzymatic reaction. The majority of the stained neurons were both NOS-IR and NADPH-d-positive, while a few others were either NADPH-d-positive or NOS-IR only. Among the co-stained neurons, four subpopulations sharing various degrees of staining intensities have been identified. These findings indicate that in the human colon a one-to-one correlation between NOS-IR and NADPH-d positivity does not exist and thus the NADPH-d reaction does not delineate with certainty all NO-producing neurons. The degree of staining intensity might account for different intracellular amounts of these two enzymes.