21 -day-old female Norwegian Hooded rats were fed a riboflavin-deficient diet for 7 weeks. A control group consisted of individually weight-matched rats fed a complete diet. Reticulocytosis was induced by phlebotomy and heme synthesis measured in a reticulocyte-rich preparation in vitro. Concentrations of circulating iron and liver stores of ferritin iron and non-heme iron were measured. Riboflavin deficiency significantly impaired the process of accumulation and maintenance of hepatic iron stores but did not appear to influence the rate of heme synthesis in an in vitro system. A primary lesion in iron metabolism in young fiboflavin-deficient rats may be at the level of iron absorption so that assimilated iron is diverted to the erythroid marrow at the expense of repleting iron stores.