When maintained in the vicinity of the lower transition temperature of their membrane lipids, rat-liver mitochondria undergo lysis as shown by the release of malate dehydrogenase, (an enzyme located within the mitochondrial matrix), in the surrounding medium. Structural changes take place in the membranes of mitochondria subjected to increasing pressure at 0 degrees C, when the pressure reaches 750 kg/cm2. Freeze-fracture electron microscopy shows the appearance of smooth areas devoid of particles in fracture faces of mitochondrial membranes, together with zones, where aggregated particles can be seen. Concurrently, a suppression of the malate dehydrogenase structure-linked latency is observed. These structural changes can be prevented by increasing the temperature at which compression is performed. The freeze-etching observations suggest that lateral phase separations occur in mitochondrial membranes subjected to high pressure. This can be explained by supposing that pressure promotes the gel-phase appearance in a lipid system and raises the transition temperature since the transition liquid crystal lead to gel is accompanied by a decrease in volume. The deterioration of mitochondria subjected to high pressure is interpreted as a result of the lateral phase separation induced by compression in the membranes. These results are discussed with respect to our interpretation of the damaging effects that hydrostatic pressure, generated by centrifugation, exerts on rat-liver mitochondria.