The F-wave velocity in the central segment (axilla to spinal cord) of the median and ulnar nerves was studied in contrast to the conventional motor nerve conduction velocity in the proximal segment (axilla to elbow) and in the distal segment (elbow to wrist). In 33 normal subjects, the F-wave velocity in the central segment (64.3 ± 6.4 m/sec for median nerve; 63.1 ± 5.9 m/sec for ulnar nerve) was about the same as the motor nerve conduction velocity in the proximal segment (63.3 ± 6.0 m/sec; 61.3 ± 6.8 m/sec) and slightly faster than that in the distal segment (56.0 ± 5.0 m/sec; 55.9 ± 5.1 m/sec). In 10 cases of Charcot-Marie-Tooth disease, slowing of the F-wave velocity in the central segment (29.0 ± 13.1 m/sec; 28.3 ± 15.5 m/sec) was comparable to that of the motor nerve conduction velocity in the proximal segment (23.1 ± 8.6 m/sec; 22.3 ± 11.6 m/sec) and in the distal segment (21.2 ± 8.0 m/sec; 20.5 ± 11.1 m/sec).