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1.1. Iron tagged with the radioactive isotope Fe59was fed in single doses, ranging from 1.8 to 120 mg., to 466 women at various stages of pregnancy, and the uptake of iron determined.2.2. As the dosage was increased from 9 to 18 mg. and beyond, the percentage of the total dose of iron found in the maternal red cells decreased, while the amount of iron taken up increased from the lowest to the highest dose.3.3. As gestation progressed, the uptake of iron increased to the extent that at thirty weeks' gestation and over, three or more times as much iron was absorbed as during the period before the fifteenth week of gestation.4.4. The therapeutic implications of these findings are discussed. The inefficiency of absorption of large doses of iron for normal pregnant women is noted.5.5. The variation in uptake of the fetus with varying dosage does not appear to be greatly different from that of the mother. Relatively little variation in the amount of radioactive iron in the red cells of the infant at birth was observed for iron administered at different periods of gestation. The red blood cell mass of the newborn infant contained on the average about 10 per cent as much of the administered iron as that of the mother. The ratio of fetal to maternal radioactive iron per milliliter of red blood cells was not greatly different from one.6.6. The relation between parity and iron uptake is discussed.