Rigidity was measured during sinusoidal passive movements of the ankle joint in 7 patients with parkinsonism. Velocity-dependent changes were observed, less marked than in spasticity and expressed in a different way in flexor and extensor muscles: a mild decrease in resistive torques at faster stretching of dorsal flexors and an increase in resistance on stretching of plantar flexors. Dorsal flexors also frequently showed shortening reactions. Passive exercises and electrical stimulation of the peroneal nerve resulted in decreased electromyographic responses to stretch, smoother passive movements and in improved voluntary contraction.