Abstract: An atrial natriuretic peptide having profound natriuretic, diuretic and hypotensive effects, characterized by a 45 amino acid residue sequence: ##STR1## having a disulphide bond between the cysteine residues which may be extracted from mammalian heart atria is described. A synthetic peptide comprising the above sequence of 45 amino acids and a synthetic 29 amino acid residue peptide corresponding to residues 17-45 of the above sequence both containing a disulfide linkage between the cysteine residues have also been prepared and shown to have similar biological activity as the native peptides. A synthetic 23 amino acid residue peptide corresponding to residues 23-29, which is the disulphide bonded ring portion of the above sequences, has been shown to have biological activity.