The response of single motor units in the first dorsal interosseus (FDI) muscle to transcranial magnetic stimulation (TMS) of the motor cortex has been assessed using the post-stimulus time histogram during weak voluntary contraction in patients with parkinsonian symptoms and in age-matched, normal subjects. Patients and subjects were required to maintain the discharge of a motor unit at a steady rate during TMS. Responses were evident in post-stimulus time histograms of motor unit discharges as single or double peaks at mean (+/- S.E.) latencies of 23.4 msec (+/- 0.7) for normal subjects and 24.9 msec (+/- 0.9) for parkinsonian patients. There were no significant differences in latency or tendency to double peaks in the responses of motor units when normal subjects and parkinsonian patients were compared. The group data showed no significant difference between the threshold TMS for modulation of the discharge of single motor units in patients and normal subjects. However, 7 of the 15 parkinsonian patients, but only 1 of 15 normal subjects, had thresholds to TMS greater than or equal to 45% of the maximum output of the magnetic stimulator. Speed of movement was measured by 3 tasks: (1) timed stand/walk/sit, (2) timed peg-board test, (3) frequency of 2-point table taps. In the parkinsonian group there was a positive linear correlation between threshold to TMS and degree of bradykinesia for each individual score and the average score on the tests of speed of movement. This was not evident for the normal group. The results are discussed in the light of current views on the mode of action of TMS. The findings are consistent with the conclusion that parkinsonian patients exhibiting pronounced bradykinesia have a lowered excitability of the motor cortex.