The hydration of human serum albumin in solutions differing in viscosity has been studied in the temperature range of 10-40 degrees C in order to determine the nature of changes in protein hydration at thermoinduced structural transitions. The NMR technique was employed to determine the effective number of protein-bound water molecules (n) considered as a parameter characterizing the protein hydration. It is assumed that the correlation time of the fraction of water molecules (n) in protein solutions is equal to that of HSA macromolecules (tau M). It has been shown that an increase in protein flexibility (tau M decrease) is accompanied by increased n and vice versa. This result confirms the interpretation of cooperative non-denaturational transition based on the dynamic model of protein behaviour suggested earlier. The effect of an increase in protein hydration with the rise in solution viscosity has been shown to be connected with a decrease in the frequency of collisions between macromolecules as well as in the decrease of the destabilizing effect of protein cavity fluctuations over a free solvent. The increase in protein hydration with rising temperature at constant frequency of collisions is explained by the enhancement of the number of water molecules oriented under the influence of charged residues as H-bonds weakened in free solvent water.