Voluntary isometric contraction is deficient in patients with cerebellar asynergia, whereas isotonic contraction is deficient in Parkinson''s syndrome, athetosis and chorea. A theory is proposed that the cerebellum and basal ganglia are antagonistic in these functions and normally in equilibrium. Mild pseudo-parkinsonism from administration of thiopropazate improved cerebellar ataxia and intention tremor, associated with increased isometric contraction. Mild cerebellar depression by diphenylhydantoin improved voluntary motor function in basal ganglia disorders, associated with increased isotonic contraction. These observations support the theory of cerebellar-striatal antagonism.