1. The effects of ingestion of diets deficient in both copper and iron on storage of these elements and on the red cell indices have been studied in rats.
2. Injection of Cu into rats whose stores of Cu had been virtually exhausted resulted in a temporary increase in the concentration of plasma Fe and depletion of the Fe stored in the liver. Storage of Fe in the spleen seemed to be affected somewhat differently from that in the liver.
3. Fe injected into Cu-deficient rats was transported to storage sites but, although the plasma Fe concentration was presumably transiently increased thereby, there was no lasting effect.
4. The hypotheses that Cu mediates in the release of Fe from ferritin and that of Osaki, Johnson & Frieden (1966) that caeruloplasmin promotes the rate of Fe-saturation of apotransferrin are discussed.