1. 1. Using micro analytical methods that allowed work on tissue aliquots of 10 mg, the oxidative ATP-resynthesis, oxygen consumption, and the activity of seven liver enzymes have been measured in total homogenates of human liver samples removed by needle biopsy. 2. 2. A significant decrease of oxidative ATP-resynthesis and oxygen consumption has been found in cirrhotic and precirrhotic patients showing gross histological liver changes, as well as in precirrhotic states with no evident microscopical involvement. This result provides new evidence of an impaired energetic liver metabolism in cirrhotic diseases. 3. 3. The activities of lactic dehydrogenase, malic dehydrogenase, isocitric dehydrogenase, succinic dehydrogenase, aldolase, DPN-cytochrome c reductase, and glutamic-oxalacetic transaminase were found to be significantly reduced with reference to a tissue wet weight basis. When calculated in terms of parenchymal protein content this reduction remained significant for aldolase and isocitric dehydrogenase. Thus 2 steps of the glycolytic chain and tricarboxylic cycle appear to be impaired in the remaining parenchyma of cirrhotic and precirrhotic livers. 4. 4. The reduction of oxidative ATP-resynthesis, oxygen uptake and aldolase activity does not seem to be specific for cirrhotic and precirrhotic states. In a group of 40 patients including noncirrhotic diseases, a significant correlation was found between the results of these in vitro determinations and the clinical evaluation of liver function. 5. 5. It is assumed that the direct in vitro measurement of an overall cellular process like oxidative ATP-resynthesis in human liver samples could provide a more accurate and more representative evaluation of the liver function than most of the usual clinical tests.