Earlier investigators observed that addition of large amounts of zinc to the diet or rats can retard growth, lower their hemoglobin levels and reduce storage of iron. In the present studies, addition of 0.75% zinc to a synthetic diet confirmed the reduced storage of iron in the livers and spleens of growing rats, but failed to show an effect on growth rate or hemoglobin levels. The adverse effects of zinc excess on growth and hemoglobin level could, however be reproduced by replacing the Rogers-Harper salt mixture used in the present studies with the Wesson salt mixture use in the earlier studies. Rats fed excess zinc along with the Wesson salt mixture grew less well, had anemia and also had low level in copper in their livers. It is suggested that addition of zinc to the low copper Wesson salt mixture reduced copper absorption sufficiently it deplete liver copper to a level at which mobilization of liver iron stores by a copper-dependent mechanism became impaired, thus depriving red cell production and tissue enzymes of stored iron. The mechanism by which iron stores are depleted by addition of zinc to the better balanced Rogers-Harper salt mixture remains unexplained It is not due to interference by zinc with iron adsorption from the diet nor with cellular uptake of iron from circulating transferrin, and the capacity of tissues to store iron as ferritin is not impaired.