Motor conduction velocity is expected to be normal or nearly normal in amyotrophic lateral sclerosis (ALS). Some studies have suggested that pathology may be present in the proximal axons. Indeed, some investigators have shown a decrease in the proximal conduction velocity in ALS by using motor conduction velocity measurements and H-reflex and F-response recordings, but they could not delineate the precise region of the conduction pathology. In this study, unlike the ones carried out previously, the most proximal segment has been studied in 11 patients with ALS, 13 normal controls, and 5 patients with sequel of poliomyelitis (SPM) by recording sensory and motor spinal root potentials. While no conduction pathology, H-reflex, or F-response abnormalities were found in ALS patients compared to normal subjects, it was shown that conduction velocity decreased in the proximal segment of the lower motor neuron of the ventral root. Despite motor neuron pathology in SPM, there was no proximal motor conduction slowing compared with that in normal subjects.