The effect of elevated hydrostatic pressure on the secondary structure of poly(L-lysine) was studied using Fourier transform ir spectroscopy. According to changes observed in the amide I band, both the β-sheet and the unordered polypeptide undergo a reversible, pressure-induced conformational change to α-helix. The conversion occurs at a much higher pressure from the unordered conformer (∼ 9 kbar) than from the β-sheets (∼ 2 kbar). The structural changes were found to be slower at pH > 11, especially at the highest concentration investigated (10 wt%), reflecting the fact that extensive hydrogen-bond networks have to reorganize. This study shows that alterations of polypeptidic conformations induced by elevated hydrostatic pressure are reversible, but that an apparent irreversibility can result from kinetic factors in the case of conformational changes involving extensive rearrangements. The present results also show that the strength of the hydrogen bonds between the backbone amide groups is not the only factor that determines the closest packing of the polypeptide molecules.