Spin-lattice (T1) and spin-spin (T2) relaxation times for protons of methyl and aromatic side groups of carbonic anhydrase B have been measured in three states: native (pH 7.2), molten globule (pH 3.3) and completely unfolded (pH 3.3, in the presence of 8 M urea). The changes of T1 and T2 were treated based on the assumption of two types of molecular motions: (1) isotropic "slow" motions with times approximately greater than 10(-8) s (including the rotation of a molecule as a whole) and (2) anisotropic "fast" motions with times approximately less than 10(-10) s. Experimental data show an essential increase of the scale of intramolecular mobility for the majority of side groups upon transition of the protein from the native to the molten globule state.