The solution structure of eel calcitonin in a mixture of 60% water and 40% trifluoroethanol has been determined in this study by the combined use of 1H-NMR spectroscopy and distance geometry calculations. 1H-NMR spectroscopy provided 181 distance constraints, 5 dihedral angle constraints and 14 hydrogen bond constraints. The observed NOEs demonstrated the presence of an amphiphilic a-helix in position Leu4-Gln20. The seven best converged structures exhibit backbone atomic rmsd of 0.027 nm for the backbone atoms from the averaged coordinate position in the region of Cysl-Leu19. In the previous study [Ogawa, K., Nishimura, S., Doi, M., Kyogoku, Y., Hayashi, M. & Kobayashi, Y. (1994) Eur: J. Biochem. 222, 659-666], the conformation of elcatonin, an analogue of eel calcitonin, was characterized by an amphiphilic a-helix between Thr6 and Thr21 and a turn structure in the first five residues of the N-terminus. The major difference of structure between eel calcitonin and elcatonin exists within the cyclic moiety at the helical N-terminus. Some of the turn structure detected at the N-terminus in elcatonin is not found in eel calcitonin. This is attributed to the difference in the ring formation caused by the disulfide bridge and ethylene bridge. A medium-range NOE, dalphaN(i,i+2), is detected between the CalphaH of Arg24 and the NH of Asp26, so a turn structure occurs in this segment. This NOE connectivity profile in the C-terminal region is also the same in elcatonin, suggesting that this is important for receptor binding and immunological properties.