Plasma levels of gamma-aminobutyric acid (GABA) were determined in 34 clinically symptomatic patients with diagnosis of affective disorder and in 20 normal controls. Lowest levels were found in patients with familial pure depressive disease and in depression spectrum disease, while levels of patients with sporadic depressive disease, though significantly lower than control, were not as low. Plasma GABA levels in secondary depression and in bipolar depression were similar to control. Bipolar manic patients had plasma GABA levels that were significantly higher than control values.