The 1H-NMR spectra of the oligosaccharide derived from monosialoganglioside GM1 (GM1 = beta-D-galactosyl-(1-3)-beta-D-N-acetylgalactosaminyl-(1-4)- [alpha-N-acetylneuraminyl-(2-3)-]-beta-D-galactosyl-(1-4)-b eta-D-glucosylceramide) (GM1OS) and its reduced form (GM1OS-R) have been obtained at 500 MHz in D2O. Through the combined use of one-dimensional and homonuclear two-dimensional spin-echo J-correlated (2D SECSY) spectra of GM1OS-R, the assignments for the ring protons of GM1OS are made. Data on chemical shifts and coupling constants of GM1OS including the alpha-linked neuraminic acid protons, in aqueous solution, are tabulated. Due to the very small coupling constants (less than 2 Hz) and the closeness in chemical shifts (less than 0.04 ppm) for the pair of correlated peaks in the two-dimensional spectrum, the information on the connectivities of the H5 ring protons of the neutral sugar residues is missing. Second-order coupling also blurs this information. Data are compared with those obtained for ganglioside GM1 in dimethyl sulfoxide (DMSO; the actual composition therein was 97% DMSO-d6 and 3% D2O) by T. A. W. Koerner, J. H. Prestegard, P. C. Demou, and R. K. Yu (1983, Biochemistry 22, 2676). While the heterogeneity of chemical shifts for the H5, H6a, and H6b protons diminishes in D2O, that for A-9a and A-9b remains. The latter suggests an intraneuraminic acid conformation involving the glycerol side chain unaffected by the solvent. Moreover, the chemical shifts of the III-1, III-2, and A-4 protons (and perhaps the II-4, IV-2, and A-8 protons) in D2O exhibit unusual upfield shifts compared with those in DMSO. This indicates that the intramolecular interactions between GalNAc residue III and neuraminic acid present in DMSO are weakened in D2O. The effect of temperature on the conformation is also examined and appears to be minimal (less than 0.02 ppm) in the range 22-50 degrees C.