Fourier transform infrared (FTIR) spectroscopy combined with resolution enhancement techniques, second-derivative and difference spectroscopies, have been used to characterize pressure-induced changes in the structural rearrangements of bovine pancreatic trypsin inhibitor (BPTI) in D 2 O solution at 25.0ЊC. According to the observed changes in the amide I band up to 550 MPa, the secondary structure elements of BPTI, such as the a-helix, 3 10 -helix, b-sheet, and b-turn, are scarcely rearranged except for the loop structure of residues of 9-17 and 36-43. The polypeptide backbone is not extensively unfolded up to 550 MPa. The minor pressure-induced structural rearrangements are completely reversible. A further increase in pressure above 1000 MPa associated with the precipitation of BPTI in D 2 O buffer solution induces the partial structural rearrangements of the a-helix, b-turn and/or 3 10 -helix, and b-sheet. The polypeptide backbone of BPTI is not fully unfolded even above 1000 MPa. Most of the protected backbone amide protons involved in the b-sheet remain intact in the pressure range where BPTI is not precipitated, while those involved in the a-helix and b-turn and/or 3 10 -helix are exchanged with solvent deuterons. The protected backbone amide protons located near the surface regions are more easily exchanged with solvent deuterons by application of high pressure than those involved in the core.