Atrial natriuretic peptide (ANP) was immunohistochemically investigated in (1) right ventricular endomyocardial biopsy specimens from 87 apparently healthy donor hearts taken from victims of cerebral accidents; (2) 1 normal heart not suitable for transplantation (HBsAg carrier); (3) right ventricular endomyocardial biopsy specimens from 151 patients with dilated cardiomyopathy (DC); and (4) 57 explanted hearts, 26 with DC and 31 with ischemic heart disease. No ANP immunoreactivity was found in normal ventricles. Failing hearts showed ventricular positivity in 31% of the DC biopsy series, in 61% of the left ventricles, and in 30% of the right ventricles of the explanted heart series. An endoepicardial gradient was observed, because ANP positivity was greater and more extensive in the subendocardial layers. Ultrastructural studies were performed on biopsy specimens from 10 normal hearts and 132 DC biopsy samples. No ANP-storing granules were found in biopsy samples of normal ventricles, whereas ANP granules were seen in 15 of 132 (11.4%) DC cases. In parallel immunoblotting, investigation showed the same 13 kDa band protein in 1 normal atrium as well as in 8 failing atria and ventricles. ANP immunoreactivity was positively correlated with higher New York Heart Association functional classes as well as with higher left ventricular end-diastolic pressure (p less than 0.005), end-diastolic volume (p less than 0.005) and end-diastolic volume index (p less than 0.005). In conclusion, apparently healthy ventricles do not show ANP immunoreactivity, whereas failing ventricles do. ANP expression seems to be independent of the underlying disease, but positively related to the clinical status and the degree of left ventricular impairment and dilatation.