Observation of individual hydration water molecules located in the solvent-inaccessible core of globular proteins by nuclear magnetic resonance (NMR) spectroscopy showed that these have identical H-1 chemical shifts as the bulk water, implicating exchange-averaging of the shifts in the two environments. Measurement of the water exchange rates for the internal hydration sites is of interest since these can be directly related to the frequency of internal motions of the protein. This paper describes measurements of the water exchange rates with an experimental scheme based on the use of pulsed field gradients. The previously established lower limit of k(ex) > 50 s(-1) for the exchange rates of the four internal water molecules in the basic pancreatic trypsin inhibitor could thus be raised to k(ex) > 1 x 10(3) s(-1), with an upper limit of k(ex) < 1 x 10(9) s(-1) established by previous investigations.