Ventricular fluid concentrations of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), the respective metabolites of dopamine and serotonin, were measured in 57 patients undergoing thalamotomy for relief of movement disorders. The diseases included were Parkinson disease, dystonia, cerebral palsy, multiple sclerosis, and posttraumatic or posthypoxic encephalopathy. Untreated parkinsonian patients had the lowest mean HVA level (119 ng per milliliter). Patients with multiple sclerosis or with posttraumatic or posthypoxic encephalopathy with both intellectual impairment and bilateral motor involvement had lower mean HVA levels (197 and 177 ng per milliliter, respectively) than cerebral palsy patients with bilateral motor disease (233 ng per milliliter), dystonia patients (246 ng per milliliter), or multiple sclerosis patients with normal intellect (376 ng per milliliter). The data suggest that diffuse cerebral disease may lead to diminished dopaminergic activity. Ventricular fluid 5-HIAA levels were similar in all groups of patients. Chronic cerebellar stimulation markedly increased ventricular fluid HVA and 5-HIAA levels, indicating that cerebellar stimulation affected cerebral dopaminergic and serotonergic systems.