1. In rats iron was absorbed after administration into the gut lumen as ferric iron bound to serum albumin, to nitrilotriacetic acid, and to 8-OH-quinoline sulfonic acid, or as isolated diferri-transferrin. 2. Iron absorption from 59Fe-labelled transferrin was inhibited by the addition of rat plasma. 3. The inhibitory component in the rat plasma turned out to be ceruloplasmin (ferrous iron oxidase, EC 1.16.2.1). 4. The absorption of iron from these ferric iron complexes was also inhibited by addition to the incubation medium of ferrozine, a strong anionic Fe(II)-ligand. 5. Uptake and absorptive utilization of transferrin-bound ferric iron was decreased after a prewash of the gut lumen and could be restored by the addition of ascorbate to the incubation medium. 6. The conclusion was drawn from these results that luminal reduction precedes ferric iron absorption and that this is a prerequisite for the uptake into the mucosa.