The activity of ornithine carbamyl transferase (OCT) and glutamate pyruvate transaminase (GPT) in serum has been correlated with the extent of necrosis 24 h after different periods of ischaemia in rat liver. The extent of necrosis has been quantified as the volume density of necrosis in the total ischaemic liver lobes using tetranitro BT. The GPT-activity in serum is maximal between 1 and 5 h after different periods of ischaemia, whereas OCT reaches its maximum between 5 and 12 h after ischaemia. The total amount of leaked enzyme-activity as well as the peak value give a linear correlation with the extent of necrosis for OCT and GPT. There is a difference between the character of these two enzymes in that a small leakage of GPT does not indicate liver cell necrosis later on. However, the appearance of OCT in the blood, an enzyme localized in the mitochondrial matrix, has a predictive value for the extent of necrosis, likely to occur later on. GPT, an enzyme from the cytoplasm, can also occur in the blood during the reversible stage of liver cell damage.