Quantitative measures of liver functions were investigated in rats up to 360 h after 90% hepatectomy and related to total hepatic DNA. Galactose elimination capacity (cytosolic phosphorylation of carbohydrate), p-nitro-anisole demethylase activity (endoplasmic drug hydroxylation) and prothrombin index (protein synthesis at rough endoplasmic membranes) were initially reduced as much as the liver weight, but recovered differently. During hepatic regeneration prothrombin index and galactose elimination were back to control values after an interval of 360 h, while p-nitro-anisole demethylase activity was about 0.4 times control value after that interval. The correlation between total hepatic DNA and liver weight, and between total hepatic protein and liver weight was 0.92 in both cases, indicating their close relationship during hepatic regeneration. Compared to earlier studies on 70% hepatectomy the recovery of metabolic functions after 90% hepatectomy is delayed, as compared to regeneration of total hepatic DNA. The compensatory hyperfunction observed after 70% hepatectomy was not found after 90% hepatectomy, indicating a lost ability to hyperfunction when hepatic function is reduced close to the minimal residual function. This is suggested to be important for the sudden onset of hepatic insufficiency.