Radioiron excretion in the urine and feces after continuous subcutaneous infusion of desferrioxamine (DF) was studied in normal and hypertransfused rats in whom reticuloendothelial and parenchymal iron stores had been labeled by selective 59Fe probes. The existence of two alternative pathways for the chelation of iron in vivo by DF was indicated by the results. The first pathway was intracellular chelation in hepatocytes with chelated iron excreted through the bile only. The second pathway, which was extracellular, could only be activated after saturation of the transferrin iron binding capacity, with the chelated iron excreted by the kidneys. The last mechanism was probably identical with the mode of action of diethylenetriaminepentaacetic acid (DTPA). Further studies will be required in order to establish whether the enhancement in iron chelation obtained by the continuous infusion of DF in patients with complete saturation of circulating transferrin may or may not be related to the extracellular mechanism of iron chelation in hypertransfused rats.