A model previously developed for describing the dynamics of flexible alkyl chains that is based on Flory’s rotational isomeric state approximation is adapted and applied to the analysis of electron spin resonance (ESR) spectra obtained from a phospholipid spin label in a macroscopically aligned phospholipid membrane, In this model, rotation around each C-C bond of the labeled alkyl chain is characterized by three inequivalent minima, with one end of the chain fixed to mimic the phospholipid headgroup, and with the dynamic effects of the nitroxide label explicitly included. This model is integrated with that for the overall rotation of the phospholipid in the mean orientational potential of the aligned membrane, and it is incorporated into the stochastic Liouville equation which describes the ESR line shape in the presence of these dynamic processes. The analysis is simplified by introducing the fact that the relatively rapid internal modes of motion can be treated by motional narrowing theory and a time scale separation can be made with respect to the much slower overall motions of the phospholipid. A series of ESR spectra from the spin label 16-PC in the lipid dimyristoylphosphatidylcholine were obtained over a range of temperatures (35-65 degrees C) in the L-alpha phase for various orientations of the normal to the bilayer plane relative to the magnetic field. Very good agreement with experiment is obtained from this model by using least squares fitting procedures for the overall motional dynamics. One finds an order parameter of [D-00(2)] that is constant throughout the phase and the perpendicular component for rotational diffusion, R-perpendicular to, that ranges from about 1-3 x 10(7)s(-1) (which corresponds to the ESR slow motional regime). Fits to the ESR spectra were also obtained from a simple but standard model wherein a single overall rotational diffusion tensor is used to describe the combined effects of internal and overall dynamics. These fits were almost as good, but they lead to a much larger R-perpendicular to approximate to 3-6 x 10(8)s(-1) and a smaller [D-00(2)] = 0.1, since these parameters now include the composite effects of both types of processes. New ESR experiments are proposed to provide more critical tests of these models.