Plasma levels of gamma-aminobutyric acid (GABA) were significantly lower in males with primary unipolar major depressive disorder than in healthy controls. Although the difference in means between control and symptomatic depressed patient groups was small, the distribution of plasma GABA in the depressed patients was markedly different from controls. Forty percent of depressed patients had plasma GABA levels below those of controls. Plasma GABA levels correlated positively with duration of illness, and negatively with age at onset of the mood disorder and the total Endogenomorphic Symptom Score on the Hamilton Rating Scale. Plasma GABA levels may be a biochemical marker of vulnerability to depression, as opposed to a consequence of the illness. A low GABA condition in depression fits and complements the prevailing biogenic amine hypotheses of depression.