1H and 13C NMR relaxation measurements at various magnetic fields have been used to characterize the nature of overall and internal motions in heparin epoxide in aqueous solution. A two-dimensional homonuclear NOESY experiment showed a considerable number of cross-relaxing protons in the molecule. The inter-proton distances calculated from NOE data were compared with those obtained by molecular mechanics calculations. Several discrepancies between the experimental and the theoretical inter-proton distances as well as the variations in 13C spin-lattice relaxation times, measured at two magnetic fields, indicated that the polysaccharide tumbles anisotropically in solution. The rates of overall and internal motions as well as the order parameters have been calculated using a model-free spectral density function. The numerical values indicate that the correlation times which characterize overall molecular motion are outside the extreme narrowing limit (tau parallel = 8 x 10(-10) s and tau perpendicular = 4.2 x 10(-8) s) and that internal motion correlation time is on a picosecond timescale.