Although brain natriuretic peptide is expressed in ventricles of failing hearts including dilated cardiomyopathy, its morphological localization is still unclear. In this study, we analyzed the immunohistocytochemical localization of atrial and brain natriuretic peptides in ventricles of dilated cardiomyopathy at both light and electron microscopic levels. Ventricular specimens were obtained by endomyocardial biopsy in 31 patients (26 with dilated cardiomyopathy and 5 controls without any specific cardiac disease). By light microscopic immunohistochemistry using specific monoclonal antibodies, all (26 of 26) of the left ventricular endomyocardial biopsy specimens and 31% (8 of 26) of the right ventricular specimens showed immunoreactivity for both of these natriuretic peptides in dilated cardiomyopathy. In contrast, none of the normal controls showed immunoreactivity for either of these peptides. The percentage of atrial natriuretic peptide-containing or brain natriuretic peptide-containing myocytes in the left ventricular specimens showed an inverse correlation with the left ventricular ejection fraction (r = -0.72 and r = -0.69, respectively). By electron microscopy, we identified specific secretory granules in ventricular myocytes from patients with dilated cardiomyopathy, but not in those from normal controls. Double immunocytochemistry using a two-face immunogold staining method revealed brain natriuretic peptide colocalized with atrial natriuretic peptide in the same ventricular granules. These findings suggest that brain natriuretic peptide is expressed in ventricular myocytes in response to hemodynamic stress in dilated cardiomyopathy. Brain natriuretic peptide may be, at least in part, synthesized simultaneously and secreted together with atrial natriuretic peptide by granules from failing ventricles, although the secretory turnover is different between these two peptides.