In 4 patients with spastic hemiparesis the electromyograms (EMG) of leg muscles were recorded during walking and the gastrocnemius medialis on both sides was investigated by histochemistry and morphometry. During walking a reciprocal mode of muscle activation was preserved on the spastic side, but the EMG amplitude was reduced. In one patient the discharge behaviour of single motor units was investigated during stance. The mean discharge rate on both the spastic and the unaffected side amounted to about 5.5 Hz. Modification of this rate over a wider range by manoeuvres of the trunk was only observed on the unaffected side. Histochemistry and morphometry of the spastic muscle revealed: Increased levels of muscle fibre atrophy (especially type II); A predominance of type I fibres during later stages, when spasticity was established; Structural changes, such as the appearance of target fibres, mainly in type I fibres. These results suggest that the low level of tonic activation in spastic muscle develops tension enough during the stance phase of gait to support the body. The histopathological profile of the spastic gastrocnemius muscle is considered to be indicative of denervation, due to the combined effects of an impaired supraspinal control of the lower motoneurone and a concurrent transsynaptic muscle membrane dysfunction, muscle cell atrophy and fibre type transformation.