Plasma glutamate decarboxylase (GAD) activity was measured in patients with endogenous psychoses and neurologic diseases. Unmedicated schizophrenic patients showed no difference in plasma GAD levels compared to controls. Administration of neuroleptics together with anticholinergic agents increased plasma GAD activity in schizophrenic patients. Compared to controls, patients with major depression and bipolar illness showed significantly lower GAD activity. No effect of antidepressants and minor tranquilizers on plasma GAD activity was found. Relatively lower GAD activity was shown in neurotic patients. The enzyme activity in plasma of patients with Huntington's chorea (HC) was lower than control levels. The plasma GAD concentrations correlated with cerebrospinal fluid concentrations in five HC patients.