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La-Lecithins having a paramagnetic V-oxyl-4', 4'-dimethyloxazolidinering at selected positions on a fatty acid chain have been prepared. A quantitative analysis of the paramagnetic resonance spectra of these “spin-labeled” lecithins in aqueous dispersions of natural phospholipids is given in terms of an effective spin Hamiltonian that results from rapid anisotropic motions of the paramagnetic/V-oxyloxazolidine ring. Good agreement is obtained between the observed and calculated resonance line shapes. Similar studies are presented for N-oxyl-4', 4'-dimethyloxazolidine derivatives of saturated fatty acids in aqueous dispersions of phospholipids and in the walking leg nerve fiber of Homarus americanus. The rapid anisotropic molecularmotion determined from the resonance spectra can be accounted for in terms of a realistic model of rapidlyinterconverting isomeric states of a polymeth-ylene chain. The analysis of …