Surface electromyograms were recorded from the quadriceps and hamstring muscles of 11 spastic patients during cyclical flexion and extension movements of the knee. A potentiometer strapped to the knee recorded the angle of the joint, the output signal being displayed on an oscilloscope. The patient used this signal to track a sine wave target for 20 cycles. The observer then moved the patient's limb through a further 20 cycles tracking the same target. Recordings were repeated at intervals for four hours after an oral dose of baclofen. Analysis of the recordings showed that the response of a spastic muscle to lengthening is not the same during passive movement as during voluntary movement. In mild spasticity stretch reflexes appear to be suppressed by voluntary effort whereas in severe spasticity they are enhanced. Baclofen suppressed the response to passive stretch by over 30% at plasma concentrations of over 250 ng/ml and by 50% at concentrations of over 400 ng/ml, but this effect was largely extinguised during voluntary movement.