Conventional peripheral motor conduction studies and transcranial magnetic stimulation (TMS) studies, to measure central motor conduction time (CMCT), to the first dorsal interosseous muscle (FDI) were performed on 65 patients with amyotrophic lateral sclerosis (ALS). The hands of each patient were classified into one of four groups depending on the presence of physical signs of lower motor neurone (LMN) and/or upper motor neurone (UMN) involvement. Statistical analysis was made of the results from patients compared with previously established normal values and with those from a control group of 53 normal subjects. Results between the four groups of patients were compared in order to assess any correlation between neurophysiological findings and physical signs. A reduction in the amplitude of compound muscle action potentials (CMAP), prolongation of distal motor latency (DML) and F wave latency were found in 36%, 34% and 19% of hands respectively. These abnormalities were more common in hands with LMN signs. In nine hands, prolongation of DML occurred in the absence of muscle wasting or weakness. CMCT abnormalities were present in 17% of patients with ALS but did not appear to correlate with physical signs.