Content of phospholipids as well as activity of phospholipase A2 and lysophospholipase A1 were studied in the hepatic mitochondrial membranes of rats exposed to high temperatures. A decrease in the content of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and increase in fractions of cardiolipin, phosphatidylserine, phosphatidic acid and lysophospholipids were detected in liver tissue mitochondria after heat stress. Alterations of phospholipid levels in the mitochondrial membranes correlated with the rate (36-37 degrees and 41-43 degrees) and duration (2 and 4 hrs) of heat exposure. Under mild conditions of heat exposure, phospholipase A2 and lysophospholipase A1 were activated in rat liver mitochondrial membranes. With elongation of the heat effect up to 4 hr activation of the lipolytic enzymes was increased. Under most severe conditions of heat stress, a marked activation of phospholipase A2 was observed, while the activity of lysophospholipase A1 was decreased and approached to control values.