The H reflex was studied in 13 patients with Parkinson's disease, and the results compared with those obtained in 22 control subjects. Pairs of identical stimuli near threshold intensity were delivered and the unresponsive period and time course of recovery of the amplitude of the H reflex were determined. The duration of the unresponsive period was significantly shorter in the patients with Parkinson's disease. In 10 patients the studies were repeated after treatment with l-dopa for six months. There was a significant increase in the period of unresponsiveness of the H reflex after treatment. The findings suggest that there is an increase in alpha motoneurone excitability in Parkinson's disease, and that it is reduced by treatment with l-dopa.