The solution structure of endothelin-1, a newly discovered potent bicyclic peptide vaso-constrictor agent, has been investigated using 1H NMR conformational constraints and distance geometry calculations. The conformation is constrained by two disulphide bridges between Cys1-Cys15 and Cys3-Cys11 but the NMR data and computed conformers show additional helical structure between residues Leu6 and Cys11. Our results are compared with previous conflicting reports on the solution conformation of this peptide.