Thyrotropin increased the steady state levels of ferritin heavy chain messenger RNA in cultured Fisher rat thyroid (FRTL5) cells by about 2.5-fold. Thyrotropin also stimulated the transcription rate of ferritin H gene determined by "nuclear run-on" assay by roughly the same extent as mRNA levels. Thyrotropin showed no effect on the stability of the ferritin heavy chain mRNA, which was suggested using actinomycin D. The results suggest that thyrotropin increases ferritin heavy chain mRNA expression in FRTL5 cells by affecting the step of transcription.