1. Oxidative phosphorylation and respiratory enzyme activities were measured in the mitochondria from the non-involved lobe of the liver in eighteen patients with massive tumour mainly localized to one lobe and from the regenerating livers of partially hepatectomized rats treated with chloramphenicol.
2. In patients, the concentrations of cytochrome a(+ a3) varied from 40 to 170 pmol/mg of protein. In mitochondria with cytochrome a(+ a3) concentrations more than 70 pmol/mg of protein, the phosphorylative activity per mg of mitochondrial protein was considerably higher than in controls. The mitochondrial oxidative and phosphorylative activities per unit of cytochrome a(+a3) were negatively correlated with the concentration of cytochrome a(+ a3). These patients with mitochondrial cytochrome a(+a3) exceeding 70 pmol/mg of protein tolerated partial hepatectomy well. On the other hand, in patients with mitochondrial cytochrome a(+ a3) less than 60 pmol/mg of protein, phosphorylative activity was very low and there was a high surgical mortality.
3. In the regenerating liver of rats treated with chloramphenicol, the oxidative and phosphorylative activities per unit of cytochrome a(+ a3) were negatively correlated with the concentration of cytochrome a(+a3).
4. It is suggested that an increase in ATP-synthesizing activity per unit of respiratory assemblies is the most basic homeostatic mechanism maintaining energy production in response to an increased metabolic load upon hepatic cells.