The F-and M-wave conduction velocities (FCV and MCV) of the right median nerve were studied in 15 normal subjects (11 males and 4 females; mean age 43.4 yr) and 25 diabetics (13 males and 12 females; mean age 46.8 yr). All the diabetics showed no symptoms or signs suggestive of radiculopathy. Supramaximal stimuli were applied to the wrist and elbow. The evoked potential was recorded by surface electrodes placed over the thenar. For measurement of the FCV, the shortest latency of several F responses was chosen. The distal FCV (cord to wrist) and MCV (elbow to wrist) in the diabetics were significantly reduced compared to the values in normal subjects. Also, the proximal FCV (cord to elbow) in the diabetics was 57.9±4.9 m/sec (mean ± SD). This value indicated a significant slowing when compared to the 62.2±3.9 m/sec in normal subjects. The above results demonstrate a pathological process of the proximal segment as well as the distal segment of the peripheral nerves in the diabetics.