The present investigation was designed to elucidate which role the sympathetic nerves play in the immunoexpression of atrial natriuretic peptide in the cardiac conduction system of the rat. In order to destroy the cardiac sympathetic nerve terminals, both surgical and chemical sympathectomy were performed. By use of immunohistochemical and radioimmunoassay techniques, the immunoreactivity and level of atrial natriuretic peptide in the conduction system and in the cardiac myocardium were determined. In contrast to the low degree of immunoreaction for atrial natriuretic peptide seen in control rats, the sympathectomized rats exhibited pronounced immunoreactivity for atrial natriuretic peptide in the atrioventricular bundle and bundle branches, which normally have high numbers of sympathetic nerve fibres. On the other hand, in the peripheral parts of the conduction system, where there are ordinarily few sympathetic nerve fibres, the degree of immunoreaction was unchanged. The quantitative measurements also showed that the entire ventricles, including the conduction system, contained increased levels of atrial natriuretic peptide in the treated hearts. The present study shows that destruction of the sympathetic nervous system leads to an increased level of atrial natriuretic peptide in the Purkinje fibres of bundle branches, which thus seem to have a dormant capacity for synthesis of this peptide. The results provide new evidence about the change in atrial natriuretic peptide levels that occurs when sympathetic innervation is altered.