Brownian and molecular dynamics simulations of a lipid bilayer are described, and the calculated frequency-dependent (13)C NMR T(1) relaxation times are compared with experiment. A consistent model emerges. Through fast internal motions, individual lipids average themselves into relatively cylindrical shapes on the 100 ps time scale and "wobble" in a cone-like potential on the nanosecond time scale. These motions take place in a highly fluid environment, much like a liquid alkane. Lateral diffusion of the lipids is on a significantly longer time scale because of restrictions at the bilayer/water interface, not because the interior of the bilayer is highly viscous.