Atrial natriuretic peptide (ANP) is present in adult atria but at very low concentrations in normal adult mammalian ventricles. In the atria, the production of ANP is regulated by physical distension of the atrial wall. The same phenomenon was investigated in the ventricles of rats and men. Cardiac tissues from human ventricular aneurysm (n = 5), spontaneously hypertensive rats (n = 30), and rats that had overloaded left ventricles induced by surgery (n = 84) were studied with the methods of light microscopic immunocytochemistry, electron microscopic immunogold staining, and RNA-RNA tissue in situ hybridization. It was found that the levels of ANP gene expression, ANP immunoreactivity, and ANP-containing specific granules in the overburdened ventricles were elevated and their degrees of fluctuation were directly proportional to the force of physical distension applied to the ventricular cardiomyocytes. In rats, ANP mRNA and ANP immunoreactivity returned to the control level seven days after the ventricular overload was surgically released. The changes of ANP and its mRNA in the ventricles were related more closely to the changes of intraventricular pressure than to cardiocytic hypertrophy. In addition, ANP immunoreactivity was demonstrated in Purkinje cells and periarteriolar cardiomyocytes in the ventricles of normotensive rats. In conclusion, physical overstretch of the ventricle wall is likely to be the triggering factor affecting ventricular cardiomyocytes to acquire endocrine property, and also to regulate the production of ventricular ANP, thereby contributing to the control of the blood volume and the blood pressure.