In the duck, intravenously injected labeled iron, in amounts that did not exceed 0.1 of the plasma iron, mixed initially with about five times the amount of iron in plasma. The highest level of labeled iron in erythrocytes corresponded to about 0.75 of the injected iron. Loss of labeled iron from erythrocytes following the maximum level indicated an average red-cell lifetime of around 40 days. The loss of labeled iron from the erythrocytes was reflected by some increase in the labeled iron of other tissues and increased loss of tagged iron by excretion. Excretion of iron was of the order of 8 x 10–4 of the total body iron per day. The appearance of the labeled iron in the cytochrome c of skeletal muscle was slow compared to the entry of the iron into the muscle and into erythrocytes.