Synthesis of the iron storage protein ferritin is induced in rat liver by iron administration, the ferritin L subunit being preferentially stimulated over the H subunit. To examine the basis for this differential regulation of the two subunits, the transcription rates of the L and H genes, the total cellular levels of L and H subunit mRNA, and the distribution of the mRNAs between a stored ribonucleoprotein form and the polysomes were examined in rat liver at several times after iron injection. Iron caused a rapid increase in transcription of the L subunit gene, followed by a rise in L mRNA levels, whereas H subunit gene transcription and H mRNA levels did not increase significantly. Differential transcriptional regulation of ferritin L and H mRNA levels by iron contrasted with the coordinate control of the two subunit mRNAs at the translational level. On giving iron, there was a rapid and synchronous shift of both mRNAs from the ribonucleoprotein fraction onto the polysomes, the same proportion of each mRNA being mobilized. Thus, regulation of ferritin subunit synthesis at these two levels allows both a rapid translational response and a specific transcriptional response to increased intracellular iron levels.