Left ventricular hypertrophy or treatment with dexamethasone caused a 2.5-fold to threefold increase in both immunoreactive atriopeptin (AP) and AP messenger RNA (mRNA), primarily in left ventricular tissue. The combined treatments increased immunoreactive AP and AP mRNA more than either treatment alone. In the animals in which cardiac hypertrophy had been produced by abdominal aortic constriction, there was a decrease in atrial levels of AP and an increase in plasma levels of immunoreactive AP. The increase in left ventricular immunoreactive AP was confirmed by immunohistochemical staining of tissue from hypertrophied and/or dexamethasonetreated rats. The mRNA accumulated in the left ventricle was identical to atrial AP mRNA, as judged by transcriptional start site and by size on Northern blots. Because the mass of ventricular tissue is substantially greater than that of atrial tissue, the induced mRNA levels may represent a total abundance approaching one third of the total AP mRNA in the atria. High performance liquid chromatographic purification of ventricular extracts primarily demonstrated the presence of the high molecular precursor and small amounts of C-terminal peptide AP. Induction of ventricular AP (mRNA and peptide) may represent regression of the tissue to an earlier developmental form. These data provide a unique example of regulation of AP biosynthesis in nonatrial tissue.© 1987 American Heart Association, Inc.