In this study we describe the ability of two human urotensin-II (hU-II) derivatives [Pen5,Orn8]hU-II(4–11) and [Pen5,DTrp7,Orn8]hU-II(4–11) (urantide) to block hU-II-induced contractions in the rat isolated thoracic aorta. Both compounds competitively antagonized hU-II- induced effects with pKB=7.4±0.06 (n=12) and pKB=8.3±0.09 (n=12), respectively. In contrast, neither [Pen5,Orn8]hU-II(4–11) nor urantide (1 μM each) was able to modify noradrenaline- or endothelin 1-induced contractile effects. At micromolar concentrations, [Pen5,Orn8]hU-II(4–11) produced weak (⩽25% of hU-II maximum) agonist responses in the rat aorta, whereas urantide was totally uneffective as agonist up to 1 μM. In addition, [Pen5,Orn8]hU-II(4–11) and urantide displaced [125I]urotensin II from specific binding at hU-II recombinant receptors (UT receptors) transfected into CHO/K1 cells (pKi=7.7±0.05, n=4 and pKi=8.3±0.04, n=4, respectively). To our knowledge, urantide is the most potent UT receptor antagonist so far described, and might represent a useful tool for exploring the (patho)physiological role of hU-II in the mammalian cardiovascular system.