The surface electromyogram (EMG) of mm. tibialis anterior and triceps surae was recorded in 10 patients with spasticity, 10 patients with rigidity and 20 normal subjects and correlated with the changes in ankle joint angle during the different phases of the gait cycle. While the strength and timing of EMG activity recorded from triceps surae during the stance phase of gait did not differ from that of normal subjects, the EMG of tibialis anterior was significantly stronger during the swing phase in both groups of patients. Although the reciprocally organized innervation pattern of the leg muscles was preserved, spastic patients could hardly lift up the affected foot during the swing phase despite the enhanced activity of tibialis anterior. There was no coactivation of the calf muscles during the hyperactivity of tibialis anterior. Therefore, no electrophysiological explanation could be found for the increased muscle tone in either group of patients. The possibilities of reduced force development by the muscle fibres of tibialis anterior or of some mechanical obstruction in the ankle joint were largely excluded as alternative explanations underlying the impeded elevation of the foot. We suppose that in both diseases the muscle fibres undergo changes which are responsible for increased muscle tone in spasticity and rigidity. The pathophysiological mechanism of these changes remains unknown.