Carbon-13 n.m.r, spectra have been obtained for bovine myelin basic protein at pD 4.4 in D2O and in 6 M guanidine deuterochloride solutions. Chemical-shift differences between resonances from some amino acid residues are interpreted in terms of structured regions in the polypeptide chain of the native protein, whereas the denatured protein displays the spectrum expected for an essentially random coil. Measurements of T1 and n.O.e. provide quantitative data on the dynamics of the backbone and side-chain carbons, and give support to the conclusion that the native protein does not have a random-coil structure.