Neutron diffraction measurements demonstrate that hydrostatic pressure promotes liquid-ordered (Lo) domain formation in lipid membranes prepared as both oriented multilayers and unilamellar vesicles made of a canonical ternary lipid mixture for which demixing transitions have been extensively studied. The results demonstrate an unusually large dependence of the mixing transition on hydrostatic pressure. Additionally, data at 28 °C show that the magnitude of increase in Lo caused by 10 MPa pressure is much the same as the decrease in Lo produced by twice minimum alveolar concentrations (MAC) of general anesthetics such as halothane, nitrous oxide, and xenon. Therefore, the results may provide a plausible explanation for the reversal of general anesthesia by hydrostatic pressure.