1. In sepsis various processes of carbohydrate metabolism, such as hepatic gluconeogenesis and glycolysis, are altered. Phosphofructokinase-1, a key glycolytic enzyme, is controlled in the long term via regulation of synthesis and degradation of the protein itself, while in the short term it is regulated by allosteric effectors, such as fructose 2,6-bisphosphate (the most potent). In the present study hepatic phosphofructokinase-1 activity as well as phosphofructokinase-2 activity and the concentration of fructose 2,6-bisphosphate were assayed to determine if they might contribute to the derangement of carbohydrate metabolism seen commonly in sepsis. 2. The levels of glycogen and fructose 2,6-bisphosphate and the activity of phosphofructokinase-1 and phosphofructokinase-2 were determined in hepatic biopsies obtained at laparotomy from six patients with and seven patients without abdominal septic foci. 3. A significant increase in plasma lactate concentration was observed in the septic patients, whereas no significant differences in tissue glycogen content or plasma glucose concentration were seen between the groups. 4. No significant change in plasma insulin concentration was observed. However, levels of the counter-regulatory hormones (glucagon, cortisol and adrenaline) were elevated in the septic patients. 5. A 60% decrease in hepatic phosphofructokinase-1 activity was seen in the septic patients. However, no significant changes in hepatic phosphofructokinase-2 activity and fructose 2,6-bisphosphate content were observed in the septic patients. 6. The present results demonstrate that the decrease in hepatic phosphofructokinase-1 activity occurring in sepsis does not appear to reflect alterations in the concentration of fructose 2,6-bisphosphate.