Using assignments of resonances in the 1H-NMR spectrum of calmodulin obtained by the use of large tryptic fragments of the molecule [Dalgarno, D. C., Klevit, R. E., Levine, B. A., Williams, R. J. P., Dobrowolski, Z., and Drabikowski, W. (1984) Eur. J. Biochem. 138, 281--289], the spectral changes which occur on Ca2+ binding to calmodulin have been examined in detail. Ca2+ binding occurs in two stages: the first two Ca2+ ions bind at sites III and IV (numbered from the N terminus) and the second two Ca2+ ions bind at sites I and II. The high-affinity binding causes perturbations of residues in both halves of the molecule, whereas binding at the two N-terminal sites only affects sidechains in that half of the molecule. The effects of binding Cd2+ to the Ca2+-binding sites of calmodulin have also been studied by 1H NMR. The cation induces spectral changes which are very similar to those seen for Ca2+, but some important differences do exist.