SUMMARYThe mean haemoglobin value of a group of rats was increased by transfusions of concentrated red cells. Absorption from a test dose of iron‐59 was significantly less than the amount absorbed by untreated control animals. The reduced absorption may be a direct result of the polycythaemia or the result of the depressed erythropoietic activity following the transfusion.Rats exposed to an atmosphere with a reduced oxygen content absorbed more iron than did rats breathing air or air with 5 p.c. CO2. Hypoxia may act directly on the intestinal mucosal cell or through an intermediary compound.