Patients (18) with spasticity due to upper motor neuron lesions, 2 patients with parkinsonism, and 5 with choreoathetosis were studied electromyographically before and after the intravenous injection of not more than 50 mg of chlorpromazine. There was a dramatic abolition of spasticity for about 2 hours in 15, and lesser effect in the remaining 3 cases. Voluntary activity was unaffected and the results were unrelated to the degree of consciousness. No effect on the involuntary movements or the reflexes was seen in choreoathetotic patients. Normal persons were unaffected. Two cases of parkinsonian tremor were temporarily relieved. This contrasts with the occasional appearance of tremor in long-term treatment with chlorpromazine. The site of action is not peripheral. The drug appears to be acting on the brainstem reticular nuclei by depressing facilitatory impulses "downward" to the spinal cord, just as it depresses the "upward" influence on the cerebral cortex.