Angiotensin II is shown by nuclear magnetic resonance (NMR) to form a complex in water at pH 4.0 with cerium(III), the ideal paramagnetic probe for Ca(2+). Paramagnetic shifts induced by the metal were used for the determination of dissociation constant and complex stoichiometry. ROESY cross-peaks and 3 J(HN-H)(alpha) coupling constants were converted into distance and angular constraints to determine the structure of the complex by molecular dynamics using the simulated annealing protocol. The complex is kinetically labile and involves the Asp-1 side chain and the Phe-8 terminal carboxylates as binding groups resembling a hairpin which has been suggested as a possible biologically active structure.