The spectral density functions of the H-2-methylene nuclei in poly(ethylene oxide) (PEO) have been determined by magnetic relaxation measurements in the 0-90 MHz range at six applied field strengths. The relaxation rates were determined in a concentration interval from 0.1 to 40 monomolal. No molecular weight dependence was observed in the 3000 less-than-or-equal-to M(w) less-than-or-equal-to 50 000 range. In the dilute ana semidilute regimes, the spectral density function is interpreted in terms of anisotropic reorientation of a hydrodynamic unit. The corresponding dimensions of the dynamic unit are similar to those of a Kuhn segment. At a concentration where the dynamic unit can no longer reorient freely, the nature of the reorientational dynamics of the polymer segments is severely altered. This point is indicative of the start of the concentrated regime. Both the spectral density curve and the effective activation energies become strongly dependent on the polymer concentration. The H-1- and H-2-NMR relaxation rates show qualitatively different molecular weight and concentration dependencies at a polymer concentration greater than the crossover concentration c**. From the determination of H-1-NMR relaxation rates in mixtures of perdeuterated and hydrogenous polymers at c(p) > c**, it becomes clear that the different behavior of H-1 and H-2 is singularly due to an additional intermolecular dipolar coupling in the case of H-1. The latter interaction is modulated on relatively long time scales (tau(c) > 10(-8) s).