A new method for the study of gastrointestinal absorption of iron in mice has been described. Phlebotomy, intravenous hemolysis, hypoxia and cobaltous ion increase iron absorption. Transfusion-induced polycythemia depresses iron absorption in mice. Iron-loading by means of Imferon also depresses iron absorption. Under the conditions described for these experiments, exogenous plasma erythropoietin has no direct effect on iron absorption.