Neutron diffraction from hydrated stacks of natural two-dimensional crystal patches of purple membrane from Halobacterium salinarum was studied as a function of pressure. Measurements in H(2)O and D(2)O permitted the determination of the distribution of water of hydration in the in-plane projection of the membrane. The main experimental difference observed between the samples at 300 MPa and atmospheric pressure was a major reorganization of the hydration around the lipid head groups and protein, associated with a protein conformational change and small reductions in lamellar (stacking) and in-plane lattice spacings, which was consistent with the compressibility of membrane-protein and lipid components.