Plasma and platelet levels of excitatory amino acids were measured in 38 psychiatric out-patients and in 19 comparison subjects; the patients had DSM-III-R diagnoses of organic mental disorders (N = 3), mood disorders (N = 15), schizophrenia (N = 13), and anxiety disorders (N = 7). The glutamate plasma levels were significantly higher in the patients with mood disorders than in the comparison group.