Hypertrophy, an increase in cell size without cell division, is a fundamental adaptive process employed by postmitotic cardiac and skeletal muscle cells. Cardiac myosins undergo an adult-to-fetal isoform transition in various models of hypertrophy. Using gene-specific cDNA probes, we show here that in the adult myocardium the mRNAs encoding the fetal (skeletal muscle type) isoforms of alpha-actin and sarcomeric tropomyosin are re-expressed within 2 days in response to pressure overload. In addition, atrial natriuretic factor mRNA, so far believed to be expressed primarily in the atria, was readily detectable in the ventricles of neonates and was induced to markedly high levels in pressure-overloaded adult ventricles. In contrast, cardiac hypertrophy produced by thyroid hormone excess was not associated with induction of the atrial natriuretic factor gene or fetal contractile protein isogenes. Furthermore, the c-fos and c-myc protooncogenes and a major heat shock protein gene (hsp70) are induced in the ventricular myocardium within 1 hr after imposition of pressure overload. These results suggest that induction of cellular protooncogenes and heat shock (stress) protein genes is an early response to pressure overload, whereas reinduction of the genes normally expressed only in perinatal life, such as fetal isoforms of contractile proteins and atrial natriuretic factor, is a later event. These two types of responses might represent the general pattern of growth induction to work overload by terminally differentiated cells that have lost the ability to undergo DNA replication.