Ferritin, the iron storage protein, was recently suggested to be a candidate reporter for the detection of gene expression by magnetic resonance imaging (MRI). Here we report the generation of TET:EGFP-HAferritin (tet-hfer) transgenic mice, in which tissue-specific inducible transcriptional regulation of expression of the heavy chain of ferritin could be detected in vivo by MRI. We show organ specificity by mating the tet-hfer mice with transgenic mice expressing tetracycline transactivator (tTA) in liver hepatocytes and in vascular endothelial cells. Tetracycline-regulated overexpression of ferritin resulted in specific alterations of the transverse relaxation rate (R(2)) of water. Transgene-dependent changes in R(2) were detectable by MRI in adult mice, and we also found fetal developmental induction of transgene expression in utero. Thus, the tet-hfer MRI reporter mice provide a new transgenic mouse platform for in vivo molecular imaging of reporter gene expression by MRI during both embryonic and adult life.