Recent data have shown that ferritin, a ubiquitous protein, has a role as a regulator of cellular differentiation. In the present study we have investigated the expression of ferritin mRNAs in cultured C6 cells, a rat glioma cell line, in response to insulin, which has an important role in cellular growth and differentiation. Insulin stimulated steady state levels of both ferritin heavy chain and ferritin light chain mRNAs. An increase in the level of ferritin heavy or light chain mRNA was detected after 2 h of incubation with insulin, and a plateau was reached after 48 h for heavy chain mRNA and after 72 h for light chain mRNA. The responses were dose-dependent and were maximal at 100 nM for both mRNAs. Treatment of cells with actinomycin-D showed that insulin had no effect on the posttranscriptional stability of these mRNAs. Actinomycin-D inhibited insulin-induced accumulation of both mRNAs, suggesting transcriptional stimulation of ferritin genes by insulin. A nuclear run-on assay showed that the insulin-induced increase in ferritin heavy chain mRNA was due to an increase in the rate of gene transcription. We also demonstrated that insulin-like growth factor-I (IGF-I) increased ferritin heavy and light chain mRNA levels in a dose-dependent fashion, and that the maximum effect was obtained at a concentration of 10 nM on both mRNA levels. IGF-I was not only 10-fold more potent, but the absolute level of maximum stimulation was also about 2-fold greater than that for insulin. The combination of insulin (100 nM) and IGF-I (10 nM) showed no additive effect. The results suggested that the ferritin heavy and light chain genes are transcriptionally regulated by insulin and influenced by IGF-I.