Sympathetic denervation of the rat heart was produced by 6-hydroxydopamine. In denervated ventricles abundant "atrial" granules were observed whereas in controls no such granules were seen. Denervated ventricles and atria showed a very high content of atrial natriuretic peptide-like immunoreactivity (greater than 10 times higher than in control animals). We suggest that the sympathetic nervous system exerts a repressive effect upon synthesis of atrial natriuretic peptide-like material and upon the formation of atrial natriuretic peptide-storing granules in the ventricular myocardium.