The uptake of iron by the absorptive cells of the first part of the duodenum of the normal mouse, and the subsequent transfer of iron to the lamina propria, have been studied by radioautography at intervals in the first 24 hr after instillation of 55Fe into the stomach. Two phases of iron absorption are seen during this time. In the first 3 hr, iron is taken up very quickly by the brush border of the absorptive cells; the iron becomes localized mainly in areas rich in rough endoplasmic reticulum and free ribosomes; and there is rapid transfer of iron to the vessels of the lamina propria without evidence of any specific cell acting as a "carrier." The second or slow phase occurs between 3 and 24 hr. The uptake of iron by the absorptive cells and its transfer to the lamina propria appears minimal. During this period the radioactive iron is found mainly close to rough endoplasmic reticulum and free ribosomes and only a relatively small amount is seen over ferritin granules in lysosomes. Significant numbers of spots, representing radioactive iron, have not been observed at any stage over the mitochondria, nuclei, and Golgi zones.