Female Norwegian Hooded rats were given a diet low in riboflavin prior to pregnancy and during gestation and lactation. Each animal had a pair-fed control receiving a diet adequate in riboflavin. The ability of liver mitochondrial preparations to mobilise iron from ferritin was measured in the dams and their pups. The rate of mobilisation was reduced in the deficient animals relative to their controls and significantly so for the pups. The data support the hypothesis that riboflavin is an important factor in controlling iron utilisation.