The three-dimensional structure of the bovine pancreatic trypsin inhibitor (BPTI) contains 4 internal water molecules, denoted W111, W112, W113, and W122, the latter being replaced by a seryl side chain in the BPTI(G36S) analogue. To investigate the effect of the exchange between these explicit water sites and the bulk solvent, we have measured water 17O and 2H nuclear magnetic relaxation in solutions of BPTI and the G36S mutant over the Larmor frequency range 2.6-49 MHz. A comparison of the data from the two nuclei shows unequivocally that the isolated buried water molecule, W122, of BPTI contributes only to 2H, but not to 17O relaxation, while the other 3 waters contribute fully to the relaxation of both nuclei. This demonstrates that the residence time of W122 is in the range 10-200 microseconds, while the residence times of W111-W113 are in the range 15 ns-1 microseconds. The slower exchange of W122 indicates that the functionally active region of BPTI, near the Cys14-Cys38 disulfide bond, is less flexible than the central region of BPTI, where the other 3 buried waters are located.