In severely depressed patients with primary affective illness, consistent decreases in total calcium concentration have been demostrated in both CSF and serum following successful ECT treatment. The hypocalcemia does not appear to occur after the initial ECTs, but develops after three to five treatments, coincident with an acceleration in clinical antidepressant effects, and is not an artifact of the anesthetic premedications or mechanical ventilation of the patients. Evidence linking alterations in calcium metabolism to mood is reviewed and a number of mechanisms by which the calcium change might result from ECT and mediated its effects on mood are discussed.