The rate of detoxication of benzoic acid was determined in patients with various neuropsychiatric diseases. About one half of the patients with catatonia showed a defect in hippuric acid synthesis. This may be due to the state of muscular rigidity and immobility of these patients. Deteriorated patients with epilepsy showed a defect in the synthesis of hippuric acid, whereas extramural patients with epilepsy showed a normal excretion of the equivalent benzoic acid. This may indicate a defect in glycine synthesis in deteriorated patients with epilepsy. Normal average values were found in post-encephalitic Parkinsonism and multiple sclerosis. A patient with progressive muscular dystrophy was able to synthesize glycine at a normal rate.