The main chain dynamics of a peptide corresponding to the smooth muscle myosin light chain kinase calmodulin-binding domain bound to calcium-saturated calmodulin have been studied by 15N relaxation techniques. Laboratory and rotating-frame spin lattice relaxation times and nuclear Overhauser effects have been determined for nine amide 15N sites in the peptide using two-dimensional NMR spectroscopy. The global motion of the 1:1 complex is shown to be isotropic and is characterized by a correlation time of 10 ns rad-1. The generalized order parameters (S2) of the nine backbone amide N-H vectors of the peptide all fall closely about a value of 0.83. The corresponding effective correlation times all tend to zero, indicating that, on the subnanosecond time scale, backbone motion of the bound peptide is highly restricted and dominated by extremely fast motions.