Experiments were performed to determine whether spinal and supraspinal components of reciprocal inhibition (a neural mechanism responsible for the prevention of muscular co-ordination during voluntary movement) were present in groups of non-disabled children and children with cerebral palsy. Changes in the gastrocnemius-soleus H reflex were examined during voluntary dorsiflexion and plantarflexion of the ankle and during a vibration applied to the anterior tibial tendon. The results indicate that children with cerebral palsy have impairments in reciprocal inhibition, both before and during voluntary movement. These deficits, which involve damage to supraspinal centres, contribute to their inability to perform smooth, co-ordinated movements.