The anisotropy of the 2H spin-lattice relaxation time T1z in oriented bilayer membranes was used to evaluate the major relaxation pathways in the MHz regime. Anal. of the expts. is achieved in terms of a d. operator formalism, employing the A. Redfield (1965) approxn. A comparison of the obsd. angular dependences of T1z with those predicted for various motional modes indicates that restricted rotational diffusion of individual mols. represents the dominant relaxation process. Computer simulations provide the orientational order parameters and motional correlation times of the lipid mols.