Synthetic natriuretic peptides (SNP) composed of 23 amino acid residues of which residues 2-23 have the natural L-configuration while the amino-terminal residue is derived from an unnatural amino acid having the D-configuration. The compounds possess diuretic, natriuretic, vasorelaxant, smooth muscle relaxant, hypotensive, and anti-hypertensive activities, and a process for their preparation is also disclosed, together with pharmaceutical preparations thereof and with their use in the practice of medicine. Salts of said synthetic natriuretic peptides with pharmaceutically acceptable acids are also disclosed.