Decreases in the infrared spectroscopic amide A frequencies from 3308 to 3291 cm(-1) of native purple membrane as a function of pressure are consistent with a gradual alpha(II)- to alpha(I)-helical conversion for the transmembrane helixes of bacteriorhodopsin. This structural transition reaches completion near 3.7 kbar. Only negligible frequency shifts are observed, however, as a function of pressure in the 1660 cm(-1) amide I region, suggesting that additional factors such as hydrogen bonding and helix-helix interactions are important in modulating the; frequency of this mode. The pressure dependence of the feature at 2927 cm(-1), arising from a Fermi resonance couplet originating from both protein and lipid methyl groups, indicates that global environmental changes occur near 4 kbar.