The phase transitions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer membranes were observed by high-pressure optical methods. The so-called main transition between the liquid crystalline (L-alpha) and the lamellar gel (L-beta) phases was observed in water at above 0 degrees C under high pressure, in addition to the transition between the L-alpha and the lamellar crystalline (L-c) phases in 50% aqueous ethylene glycol. The pressure of the main transition increased linearly with an increase in temperature. Extrapolation of temperature (T) - pressure (p) phase boundary to ambient pressure suggests the temperature of the main transition to be -40.3 degrees C, which has never been found by DSC methods. On the other hand, the temperature of the L-c/L-alpha phase transition in 50% aqueous ethylene glycol was found to be -12.0 degrees C at ambient pressure. The values of dT/dp were 0.233 K MPa-1 for the L-beta/L-alpha phase transition and 0.147 K MPa-1 for the L-c/L-alpha phase transition, respectively.