Abstract:The electrophysiological examination was made to clarify the effect of ethambutol on the peripheral nervous system in 39 patients at tuberculous sanatorium. Abnormalities in the sensory nerve action potential (toe‐ankle) were observed in about 10 percent of the patients treated with ethambutol, and these patients were mostly elderly and/or received the high dose of ethambutol. The mixed nerve conduction velocity (ankle‐knee) showed a significant decrease compared with healthy subjects. The conduction velocity tended to decrease in the high ethambutol dosage (200 gm or more) group compared with the low dosage (200 gm or less) group. The tendency for the conduction velocity to decrease was also observed in the 18 patients treated with isoniazid but the abnormalities were mild compared with the ethambu