1. The mechanism of the stimulation of ferritin synthesis by iron in vivo has been studied in rat liver. Ferritin synthesis and turnover was measured by [14C]leucine incorporation. 2. Actinomycin D had no inhibitory effect, after administration of iron, on [14C]leucine incorporation into ferritin but appeared to augment the effect of iron on ferritin synthesis. 3. Cycloheximide completely abolished the stimulation by iron of [14C]leucine into ferritin and was subsequently utilized to show that iron acts in vivo by translational induction of apoferritin synthesis, rather than by stabilization of apoferritin or its precursors. 4. This conclusion was confirmed by showing that 2 days after acute bleeding, when iron was in the process of being removed from hepatic ferritin stores, ferritin synthesis was decreased whereas breakdown rates were unchanged.
