We determined the structure in solution of the vaso-constrictor hormone urotensin II (dodecapeptide) using nuclear magnetic resonance spectroscopy. Complete assignment of all proton resonances has been achieved and the structural information has been obtained from the interproton distance measurements derived from the nuclear Overhauser enhancement data. A combination of distance geometry and dynamical simulated annealing techniques was used to calculate the structure in solution. Nine resultant structures with fewer distance constraint violations were selected that satisfy the experimental restraints very well. The conformation of the molecule in the cyclic hexapeptide segment (core region) is well-defined whereas the N-terminal segment is disordered. This result correlates very well with the earlier predictions about the biologically active and inactive roles played by the core and the N-terminal segment respectively.