The atrium of the heart contains peptides, termed atrial natriuretic factors ( ANFs ), diuretic and smooth-muscle-relaxing activities. In view of its potent effects on salt metabolism in the kidney and on vascular smooth muscle, ANF is considered to play an important part in the control of fluid volume and vascular function. Several different ANF peptides varying in size have been isolated and their amino acid sequences determined. Analysis of the sequences of the peptides suggests that they are derived by proteolysis from the same precursor. To examine this hypothesis, we have cloned cDNAs of the ANF precursor using rat atrial mRNA, determined its nucleotide sequence and deduced its amino acid sequence. The ANF precursor consists of 152 amino acid residues including a putative signal peptide sequence. This sequence contains the amino acid sequences of all the ANF peptides reported to date.