alpha-Crystallin, an oligomeric protein in vertebrate eye lens, is a member of the small heat-shock protein family. Several papers pointed out that its chaperone-like activity could be enhanced by increasing the temperature. We demonstrate in the present study that structural perturbations by high hydrostatic pressures up to 300 MPa also enhance this activity. In contrast with temperature-induced changes, the pressure-induced enhancement is reversible. After pressure release, the extra activity is lost with a relaxation time of 2.0 +/- 0.5 h. Structural alterations contributing to the higher activity were studied with IR and fluorescence spectroscopy, and light- scattering measurements. The results suggest that while the secondary structure barely changes under pressure, the interactions between the subunits weaken, the oligomers dissociate, the area of accessible hydrophobic surfaces significantly increases and the environment of tryptophan residues becomes slightly more polar. It seems that structural flexibility and the total surface area of the oligomers are the key factors in the chaperone capacity, and that the increase in the chaperone activity does not require the increase in the oligomer size as was assumed previously [Burgio, Kim, Dow and Koretz (2000) Biochem. Biophys. Res. Commun. 268, 426-432]. After pressure release, the structure of subunits are reorganized relatively quickly, whereas the oligomer size reaches its original value slowly with a relaxation time of 33 +/- 4 h. In our interpretation, both the fast and slow structural rearrangements have an impact on the functional relaxation.