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2H NMR spectra were obtained for dimyristoylphosphatidylcholine multilamellar dispersions, perdeuterated in the fatty acid chains, in the presence and absence of two steroid analogs. The presence of the active anesthetic steroid alphaxalone results in consistently smaller 2H quadrupolar splittings (delta nu Q) for individual C2H2 segments of the fatty acid chains, indicating increased molecular disorder. In contrast, the inactive analog delta 16-alphaxalone causes no significant change in the 2H spectra of the phospholipid. The conformational analysis of alphaxalone and delta 16-alphaxalone in solution was carried out with the help of 1H and 13C high resolution NMR spectroscopy and the results were used to propose a molecular model for the interaction of the two steroids with membrane phospholipids. The model correlates the observed differences in the manner in which the two steroids interact with model membranes with differences in their respective conformations and provides a molecular basis for anesthetic steroid activity.