Atrial natriuretic peptide (ANP) which is secreted from atrial muscle has been shown to produce relaxation of vascular smooth muscle, anti-hypertensive effects, and natriuresis, and to implicate renin-angiotensin-aldosterone network. In human, ANP (hANP) exists as three subtypes: alpha-, beta-, gamma-hANP which have molecular weight of 3,000, 6,000 and 13,000, respectively. In human tissue hANP has been shown to be present not only in heart atria, but also submandibular gland. In addition, it has been reported that in some conditions of heart disease hANP can be found in the ventricles. However, the exact distribution outside of the atrial system has not been definitely established. In our studies, we attempted by using immunohistochemical methods on human and dog hearts to answer this question. In addition, by using nucleic acid probes we have investigated possible areas where hANP may be synthesized. In the initial studies specific antibody to hANP was prepared in rabbits by immunization with synthetic alpha-hANP coupled to porcein thyroglobulin. The specificity of this antibody was confirmed by Western Immunoblotting. Immunoperoxidase staining demonstrated hANP in His-bundle and major branching bundles as well as atrial wall and AV-node of both human and dog hearts. Ventricular muscle cells outside of the conduction system did not contain hANP. The possibility of non-specific staining by antibody to thyroglobulin was muled out as antibody to thyroglobulin alone never showed positive staining. However, staining of the atrial muscles was always granular in the perinuclear areas, while that of the conduction system was usually diffuse.(ABSTRACT TRUNCATED AT 250 WORDS)