Heat loss during anaesthesia and surgery is a common problem. In patients with restricted cardio-pulmonary reserves this may endanger the postoperative outcome. In order to compare thermal balance we studied 25 men undergoing transurethral resection of the prostate (TURP), using either general anaesthesia (G.A.) or epidural analgesia (E.A.). Oxygen uptake, catecholamines, peripheral and central temperatures were followed in the per- and postoperative period. Heat production and total body heat were calculated from oxygen uptake and temperature measurements, respectively. Transurethral resection of the prostate resulted in a peroperative heat loss which was not influenced by the anaesthetic technique used and averaged 370 kJ during the first hour of surgery. G.A. reduced heat production while this was uninfluenced by E.A. After termination of general anaesthesia, oxygen uptake and plasma catecholamines increased, while no such changes could be detected using epidural analgesia. The ability to increase mean body temperature by increasing heat production was negatively correlated to age.