Longitudinal relaxation times T1 of water protons were measured in 5% protein solutions at different static magnetic fields (0.47, 2, and 7 T), for proteins with molecular weight ranging between 1.4 and 480 kDa and in solvents of varying degrees of deuteration. T1, values were also obtained for rat liver soaked with Krebs‐Ringer solutions of varying degrees of deuteration at the above fields. For the samples containing D2O, T1 for deuterium was also measured at fields 2 and 7 T. The deuterium measurements were used to estimate water rotational correlation times which were in turn used to estimate the contribution of so‐called “hydrodynamic effects” of macromolecules to proton relaxation. The proton relaxation rates at full deuteration were compared with those in protonated solvent (water) to obtain a second, direct measurement of this effect. Both measurements provide quantitation of the hydrodynamic effects, free from the contributions of other effects that are transparent to deuteration, and results from both measurements agree with each other reasonably well. The cross‐relaxation rate between solute and solvent protons, and the contribution of paramagnetic impurities in the samples were also obtained from the proton T1studies. The experimental results show that the hydrodynamic effects (intramolecular and intermolecular water‐water interactions) are about the same magnitude in all the proteins studied as well as in rat liver. However, the cross‐relaxation rate generally increases with increasing protein molecular weight. Measurements in soaked rat liver indicate that the cross‐relaxation rate per unit mass of solute is much higher in tissues than in simple solutions of proteins of similar mean molecular weight. The results challenge the prevailing concept that the relaxation properties of biological tissues may be treated as a simple superposition of the properties of their Constituents.