The effect of aprotinin on metabolic changes in blood of 23 patients undergoing Y-graft aortofemoral bypass operation was studied before clamping and after declamping. The right femoral artery was clamped for 122 ± 15 (–aprotinin) and 115 ± 10 min (aprotinin group), the left femoral artery 129 ± 27 (-aprotinin) and 159 ± 25 min (aprotinin group), respectively. Aprotinin caused a significant decrease of blood lactate, pyruvate, citrate and oxoglutarate levels compared with untreated controls, whereas acetoacetate and β-hydroxybutyrate concentrations were significantly increased. It is hypothesized that blood lactate and pyruvate levels are lowered due to increased pyruvate oxidation. Limitation of citrate synthesis due to lack of oxaloacetate might explain subsequent formation of ketone bodies.