The interaction between the peptide corresponding to the calmodulin-binding domain of the smooth muscle myosin light-chain kinase and (Ca2+)4-calmodulin has been studied by multinuclear and multidimensional nuclear magnetic resonance methods. The study was facilitated by the use of 15N-labeled peptide in conjunction with 15N-edited and 15N-correlated 1H spectroscopy. The peptide forms a 1:1 complex with calcium-saturated calmodulin which is in slow exchange with free peptide. The 1H and 15N resonances of the bound have been assigned. An extensive set of structural constraints for the bound peptide has been assembled from the analysis of nuclear Overhauser effects and three-bond coupling constants. The backbone conformation of the bound peptide has been determined using these constraints by use of distance geometry and related computational methods. The backbone conformation of the peptide has been determined to high precision and is generally indicative of helical secondary structure. Nonhelical backbone conformations are seen in the middle and at the C-terminal end of the bound peptide. These studies provide the first direct confirmation of the amphiphilic helix model for the structure of peptides bound to calcium-saturated calmodulin.