The effect of ascorbic acid and ethylene diamine tetra acetic acid (EDTA) in the treatment of manic-depressive psychosis was compared, using double-blind procedures, with recognized treatment regimes. There was no significant difference between the response of depressed patients to amitriptyline or ascorbic acid and EDTA. Manic patients responded significantly better to lithium than to ascorbic acid and EDTA. These results are in keeping with the suggestion that vanadium may be of aetiological importance in depressive psychosis, but do not support such a suggestion for mania.