The effect of 3,5,3'-triiodo-L-thyronine (T3) on the steady state levels of ferritin heavy chain (ferritin H) mRNA in cultured rat glioma C6 cells and various rat tissues was examined. Addition of T3 to cultured C6 cells showed the time and dose-dependent increase in the steady-state level of ferritin H mRNA. In vitro nuclear run-on assay revealed that the stimulatory effect was due to the increase in the transcription rate of ferritin H gene. T3 had no effect on the half life of ferritin H mRNA. In hyperthyroid rats, the level of ferritin H mRNA in the kidney was elevated. On the contrary, that was decreased in hypothyroid rats. The results suggest the involvement of T3 in the regulation of ferritin H gene expression.