We studied different motor evoked potential parameters: cortical threshold (CT), primary response (PR) latency and amplitude and central conduction time (CCT) at rest and after maximal voluntary contraction of the target muscles, silent period (SP) and late muscular responses (LMR) in 21 normal subjects 42- to 75-years old and compared the results to those of 17 patients with motor disease (10 amyotrophic lateral sclerosis, ALS and 7 primary lateral sclerosis (PLS). We report for the first time in patients affected by motor diseases, LMRs were similar to controls. We found abnormalities in both ALS and PLS: enhanced cortical threshold, reduced PR amplitude during maximal voluntary contraction of target muscles compared to rest and shorter SP. CCT was slightly different: in ALS it was normal or undetermined (because of lack of the PR), while it was normal, enhanced or undetermined in PLS. We conclude that except for LMR, additional parameters would be useful for studying motor diseases: CT (reflecting excitability of the central cells), PR amplitudes and SP (disclosing inhibitory pathway disturbances in pyramidal tract diseases).