There are indications that mood disorders may be related to perturbations in the amino acid transmitters. The amino acids may thus be targets of treatment of depression. The purpose of this pilot study was to measure the acute effects of a single administration of electroconvulsive therapy (ECT) on the plasma levels of amino acids in depressed patients. ECT was administered to 10 patients with major depressive disorder. Altogether 23 plasma amino acids were analyzed before and at 2, 6, 24 and 48 h after ECT. The levels of glutamate and aspartate increased at 6 h after ECT compared with the baseline. Also the levels of total tryptophan increased 2-24 h after ECT. There were also elevations in other amino acids at 6 and 24 h. The levels of gamma-aminobutyric acid (GABA) decreased at 2 h. In this study the acute effects of single ECT were associated with changes in the levels of glutamate, aspartate, GABA, tryptophan and some other amino acids. The preliminary data suggest that the therapeutic effects of ECT in depression may be due to mechanisms involving these amino acid transmitters.