We assessed the hypothesis that the epinephrine surge present during sepsis accelerates aerobic glycolysis and lactate production by increasing activity of skeletal muscle Na⁺-K⁺-ATPase. Healthy volunteers received an intravenous bolus of endotoxin or placebo in a randomized order on two different days. Endotoxemia induced a response resembling sepsis. Endotoxemia increased plasma epinephrine to a maximum at t = 2 h of 0.7 ± 0.1 vs. 0.3 ± 0.1 nmol/l (P < 0.05, n = 6-7). Endotoxemia reduced plasma K⁺ reaching a nadir at t = 5 h of 3.3 ± 0.1 vs. 3.8 ± 0.1 mmol/l (P < 0.01, n = 6-7), followed by an increase to placebo level at t = 7-8 h. During the declining plasma K⁺, a relative accumulation of K⁺ was seen reaching a maximum at t = 6 h of 8.7 ± 3.8 mmol/leg (P < 0.05). Plasma lactate increased to a maximum at t = 1 h of 2.5 ± 0.5 vs. 0.9 ± 0.1 mmol/l (P < 0.05, n = 8) in association with increased release of lactate from the legs. These changes were not associated with hypoperfusion or hypoxia. During the first 24 h after endotoxin infusion, renal K⁺ excretion was 27 ± 7 mmol, i.e., 58% higher than after placebo. Combination of the well-known stimulatory effect of catecholamines on skeletal muscle Na⁺-K⁺-ATPase activity, with the present confirmation of an expected Na⁺-K⁺- ATPase-induced decline in plasma K⁺, suggests that the increased lactate release was due to increased Na⁺-K⁺-ATPase activity, supporting our hypothesis. Thus increased lactate levels in acutely and severely ill patients should not be managed only from the point of view that it reflects hypoxia.