The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature pressure phase diagram. The large portion of the lamellar gel (L-beta'), ripple gel (P-beta'), and pressure-induced gel (L beta I) phases exist as metastable phases due to the extremely stable subgel (L-c) phase. The stable L-c phase enables us to examine the properties of the L-c phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L beta I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L-c phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (lambda(min)'') values changed with pressure, indicating that the L-c phase has highly pressure-sensible structure. The lambda(min)'' values of the L-c phase spectra were split into ca. 430 and 500 nm in the L beta I phase region, which corresponds to the formation of a interdigitated subgel L-c (LcI) phase. Moreover, the phase transitions related to the L-c phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L-c phase, we concluded that the structure of the L-c phase is highly probably a staggered structure, which can transform into the LcI phase easily.