We used peristimulus time histograms (PSTHs) to estimate characteristics of the composite excitatory postsynaptic potentials (EPSPs) generated at the anterior horn cell by a descending cortical volley induced by subthreshold transcranial magnetic stimuli in 11 normal subjects, 9 patients with DNA-confirmed Kennedy's disease, and 10 patients with amyotrophic lateral sclerosis (ALS). In Kennedy's disease the mean of the estimated EPSP amplitude (3.5 +/- 1.6 mV), as well as the means of duration (4.1 +/- 1.3 ms) and onset latency (20.4 +/- 2.7 ms) of the primary peak of 35 different motor units were not significantly different from normal subjects, in whom corresponding values of 49 motor units measured 2.9 +/- 1.6 mV (EPSP amplitude), 3.5 +/- 1.4 ms (duration) and 19.3 +/- 2.8 ms (onset latency). In 48 motor units of patients with ALS the mean of the estimated EPSP amplitude, duration, and latency measured 2.7 +/- 1.8 mV, 7.8 +/- 5.7 ms, and 24.0 +/- 6.5 ms, respectively, differing significantly from normal subjects and patients with Kennedy's disease. The normal cortically induced EPSP in Kennedy's disease is very different from the range of EPSP abnormalities in ALS, indicating that lower motor neuron disease per se does not result in an abnormal cortically induced EPSP derived from PSTHs. The normal findings in Kennedy's disease support our previous suggestions that the EPSP abnormalities in ALS are supraspinal in origin.