A semi-quantitative study was performed to clarify the distribution of substance P in the myenteric and submucous plexuses of the rat gastrointestinal tract, using the unlabeled substance P antibody-peroxidase-antiper-oxidase (PAP) method. In both plexuses, substance P-like immunoreactivities (SPLIs) were mainly found around the nerve cells.In addition, these SPLIs were localized in the structures identical to the terminal axons or varicosities; some of which were present in close proximity with nerve cell bodies, suggesting that a release of this peptide might exert some effect on the neurons. Counting of these SPLI-containing nerves in the plexuses, based on our tentative criteria, indicated a reasonable distribution ; SPLI-containing nerve was highly condensed in the upper regions of the stomach and the ileum, and gradually decreased towards the duodenum.In the esophagus, SPLI-containing nerves were rarely detected.In cats, cervical ligation of the right vagus significantly decreased the number of SPLIs in the myenteric and submucous plexuses in the ileum, and SPLIs accumulated in the proximal ends of the ligation. The results suggest that a considerable number of SPLIs in the gut might occur in the central nervous system and flow centrifugally throughout the vagus.