The aim of the present study is to determine the presence and distribution of nitric oxide (NO) producing neurons in the rat corpus callosum (CC) using nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and neuronal NO synthase (nNOS) immunocytochemistry. Intense NADPH-d positive neurons were present along the rostrocaudal extention of the CC. Neurons were more numerous in the lateral CC to gradually decrease in its medial regions, where they were very few or absent. Golgi-like appearance of NADPH-d positive intracallosal neurons allowed us to divide them into five different morphological types: i) bipolar; ii) fusiform, iii) round; iv) polygonal; v) pyramidal. Numbers of NADPH-d positive neurons was calculated in two cases. In the first case count was made in 145 sagittal sections, 60 mm thick (both hemispheres for a total of 8,700 mm of thickness) and neurons amounted to 2959; in the second case neurons amounted to 2598 and count was performed in 120 sagittal sections, 50 mm thick (both emispheres; for a total of 6,000 mm of thickness). Many of these neurons were associated with blood vessels. In addition NADPH-d positive nerve fibers were present throughout the CC. Distribution of nNOS immunopositive neurons coincides with that of NADPH-d neurons showing the same lateromedial gradient. Further analysis of ultrathin sections to study the ultrastructural organization of intracallosal NO-producing neurons are in progress in our laboratory.