Changes in the cardiac contents of microsomal RNA and poly(A)-containing mRNA have been examined during induction and regression of heart muscle hypertrophy in rat hearts, as well as possible changes in the subcellular distribution and protein-synthetic activity of cardiac mRNA. In addition, cardiac biopsies from patients with mitral valve diseases were evaluated for their mRNA contents. Induction of heart muscle hypertrophy was accompanied by substantial increases in cardiac microsomal RNA (30-60%) and cardiac mRNA (20-80%). During regression of hypertrophy increased levels of cardiac microsomal RNA and mRNA returned to normal values within 10-16 days. In general, cardiac mRNA levels were lower in human heart muscle than in rat heart muscle. Since the subcellular distribution of the microsomal RNA and of the mRNA as well as the protein-synthetic activity of the mRNA were not changed in the hypertrophied animals as compared with normal animals, and since the cardiac contents of most specific cardiac mRNA species (mRNAs for MHC, MLC1 and MLC2, actin, tropomyosin, troponin-T, myoglobin) increased proportionately, it is concluded that during induction of hypertrophy activation of gene expression occurs and affects the genes coding for the major cardiac proteins to a similar extent. This does, however, not exclude the possibility of more specific shifts in isoprotein patterns and in the levels of their corresponding mRNAs. It is proposed that changes in cardiac mRNA levels are the major regulatory factor in causing changes in cardiac protein synthesis rates leading to the induction and regression of cardiac hypertrophy.