Weanling male rats were made anemic and fed diets supplemented with 20 ppm iron and/or 2 ppm molybdenum. A decrease in serum iron was observed in the rats supplemented only with iron and a significant decrease in hemoglobin was observed in rats given no supplementation. In a second experiment, pregnant female rats were also fed diets supplemented with 20 ppm iron and/or 2 ppm molybdenum. An inverse relationship was apparent between iron and copper in both the serum and the liver of the female rats. The livers of their pups displayed an inverse relationship between molybdenum and copper. Hemoglobin in both dams and pups tended to decrease when: (1) supplemental molybdenum was absent but supplemental iron was present; (2) supplemental iron was absent but supplemental molybdenum was present; and (3) no supplementation was given at all. While there appeared to be little placental transfer of molybdenum, iron and copper seemed to be transferred from the dams' liver. In a third experiment, serum was collected from pregnant women in first, second, and third trimester, and at postpartum. Both serum iron and serum molybdenum decreased significantly at postpartum. Inverse relationships were apparent between (1) serum iron and serum copper, and (2) serum molybdenum and serum copper.