The solution structure of Ca2+ ligated calmodulin and of its complex with a 26-residue peptide fragment of skeletal muscle myosin light chain kinase (skMLCK) have been investigated by multi-dimensional NMR. In the absence of peptide, the two globular domains of calmodulin adopt the same structure as observed in the crystalline form. The so-called 'central helix' which is observed in the crystalline state is disrupted in solution. 15N relaxation studies show that residues Asp78 through Ser81, located near the middle of this 'central helix', form a very flexible link between the two globular domains. In the presence of skMLCK target peptide, the peptide-protein complex adopts a globular ellipsoidal shape. The helical peptide is located in a hydrophobic channel that goes through the center of the complex and makes an angle of approximately 45 degrees with the long axis of the ellipsoid.