In order to characterize the neurochemical features of nitrergic myenteric neurons in the rat esophagus and to disclose some possible cause of nitrergic neuronal cell loss in the rat esophagus during aging, whole-mount preparations from different esophageal segments of different age- and strain-groups were studied by double immunohistochemistry of neuronal nitric oxide synthase (nNOS) and calbindin (CB). We found for the first time co-localization of nNOS and CB in myenteric neuronal bodies with a specific distribution pattern along the entire length of rat esophagus: significantly higher percentage of nNOS/CB immunoreactive (ir) myenteric neurons (ca 30%-40%) were found in the abdominal segment in comparison with the other segments of the organ (P<0.001). Nitrergic myenteric neurons in the rat esophagus could be subdivided at least into two groups: nNOS+/CB+ and nNOS+/CB-. Moreover, the majority of CB-ir myenteric neurons,especially in the abdominal esophagus expressed nNOS-ir (ca 95%-100%). All these features were persisted in all age and strain groups of rats studied. The relative percentages of the nNOS-/CB-ir neurons among the total nitrergic population varied strain-dependently during aging. Regression analysis yielded a significant negative correlation between the percentage of nNOS-/CB-ir neurons found in the young Sprague-Dawley rats and percentage of nitrergic neuronal loss during aging (correlation coefficient 0.99; P< 0.05). However, the functional significance of the regional differences in nNOS-/CB-ir neurons along the rat esophagus remains to be elucidated.