1. As a group, manic patients have higher, and depressive patients have lower blood iodine levels than a group of normal controls.2. In individual cases of true manic-depressive psychoses, we find a close correspondence between changes in mood and changes in blood iodine.3. In contrast to these findings, we find no coincidence between changes in mood or motor excitement and blood iodine in other psychotic states.4. The significance of the findings in the pathogenesis of true manic-depressive psychoses is discussed.