Homonuclear 3D 1H NOESY-TOCSY and 3D 1H ROESY-TOCSY experiments were used to resolve and assign nuclear Overhauser effect (NOE) cross peaks between the water signal and individual polypeptide proton resonances in H2O solutions of the basic pancreatic trypsin inhibitor. Combined with a novel, robust water-suppression technique, positive and negative intermolecular NOEs were detected at 4 degrees C. The observation of positive NOEs between water protons and protein protons enables more precise estimates of the very short residence times of the water molecules in the hydration sites on the protein surface.