Human Urotensin-II is a potent vasoconstrictor and binds with high affinity to GPR14 receptor, recently cloned and renamed UT receptor. U-II vasoconstrictive potency is reported to be an order of magnitude greater than that of endothelin-1 (ET-1), which would make it the most potent mammalian vasoconstrictor identified to date. Urotensin-II is a neuropeptide "somatostatin-like" cyclic peptide, which was originally isolated from fish spinal cords, and which has recently been cloned from human. Human U-II is composed of only 11 amino acids residues, while fish and frog U-II possess 12 and 13 amino acids residues, respectively. The cyclic region of U-II, which is responsible for the biological activity of the peptide, has been fully conserved from fish to human. This review focuses on recent structure-activity relationships studies performed on Urotensin-II with the aim to provide the required structural elements to design new ligands as agonists and antagonists for UT receptor.