We have investigated the time course of the coagulation and fibrinolytic changes during moderate surgical trauma (elective reduction mammoplasty) in the absence of other confounding factors that could affect haemostasis. Specific markers for coagulation (prothrombin fragment 1.2 (F1.2), thrombin-antithrombin III complex (TAT)) and fibrinolysis (plasmin-antiplasmin complex (PAP) and D-dimer) were examined. Blood samples were obtained in 20 ASA I anaesthetized female patients at T0 (before operation), T75 (during operation) and T150 (before the end of operation). There was a progressive increase in blood loss during operation:mean 110 (SD 80) ml at T75 and 470 (180) ml at T150. This was associated with a significant increase in plasma concentrations of F1.2, PAP and D-dimer at T150 only (P < 0.05 vs T0). We conclude that moderate surgical trauma with blood losses greater than 300 ml can activate thrombin generation and fibrinolysis during operation.