There was a significant decrease in glutamine synthetase (GMS) activity between schizophrenics and controls and manic-depressives and controls in erythrocyte membrane, platelet and plasma fractions. In contrast, the GMS activity in depressives showed no significant change. Further the manic-depressives demonstrated higher GMS activity than the schizophrenics. On the other hand glutaminase (GM) activity was also decreased significantly in depressives as well as in manic-depressives but not in schizophrenics. As with GMS, the manic-depressives had higher GM activity than the depressives. Finally, no notable changes in the GTP activity were observed in any group indicating that the glutamate-α-ketoglutarate equilibrium is not disturbed.