Using the F wave, a simple equation was devised to calculate the ratio between motor nerve conduction time from the spinal cord to the stimulus site and that of the remaining nerve segment to the muscle (F ratio). In 33 healthy subjects the F ratio (mean +/- SD) was close to unity for the median nerve (1.04 +/- 0.09) with stimulation at the elbow and for the peroneal (1.11 +/- 0.09) and tibial nerves (1.17 +/- 0.10) with stimulation at the knee. Thus, in these nerves, the time required for the passage of impulses from the cord to the site of stimulation is approximately the same as that from the stimulus site to the muscle. The ratio was significantly more for the ulnar nerve (1.40 +/- 0.11) with stimulation below the elbow. Of 126 nerves in the upper and lower extremities from 45 patients with Guillain-Barré syndrome, the F ratio was normal in 65 (51%), increased in 30 (24%), and decreased in 31 (25%). The mean F ratios remained normal in median (1.12 +/- 0.40), ulnar (1.38 +/- 0.30), peroneal (1.07 +/- 0.25), and tibial (1.12 +/- 0.20) nerves. These findings together with the results of nerve conduction studies indicate that the conduction abnormality usually affects both proximal and distal segments in the Guillain-Barré syndrome. If selective, it is distributed at random between the two segments, but there is a tendency toward involvement of common sites of compression and the most proximal, possibly radicular, portion of the nerve.