A distinct distributional pattern of natriuretic peptide receptors has been demonstrated by in vitro quantitative autoradiography in the heart of Anguilla anguilla where saturable, stable, and specific binding sites for rat atrial natriuretic peptide in atrium, ventricle, and bulbus arteriosus have been detected. Competition experiments in the presence of either rat atrial natriuretic peptide or unlabeled eel peptides, i.e., atrial natriuretic peptide, C-type natriuretic peptide, and ventricular natriuretic peptide, showed differences in the relative affinity orders of these peptides in the various heart chambers. In the atrium and in the ventricular myocardium most receptors likely are type C , while B-type receptors are present in significant amounts in the ventricular endocardium and in the inner and outer layers of the bulbus arteriosus. Scatchard analysis of the competitive data indicates that in the atrium and in the ventricular myocardium only one class of receptors for rat atrial natriuretic peptide is present (& = 105 & 16 and 107 2 15 pM, respectively), while the ventricular endocardium and the inner and outer layers of the bulbus arteriosus contain two classes of binding sites (6 3.7 < & < 20 2 3.9 pM for site 1; 148 2 72 < & < 271 2 52 pM for site 2).