Expression of P-glycoprotein (P-gp) was detected by immunohistochemical staining and Western blot analysis in the peripheral nerves (7th and 8th nerves) of the guinea pig using anti-P-gp monoclonal antibody C219. P-gp was detected in the capillary endothelial cells of these nerves. Immunoreactivity in these nerves was similar to that in the brain. Besides these nerves, positive staining was observed in the sciatic nerve, although immunoreactivity was somewhat lower than that of the 7th and 8th nerves. The present investigation suggested that P-gp in the endothelial cells of the peripheral nerves might play a very important role as a part of the blood-nerve barrier function, since P-ag acts as an extension pump in these cells.