Effects of anaesthesia on serum concentrations of thyroid hormones during and soon after abdominal surgery were examined in 29 patients undergoing cholecystectomy (n = 22) or removal of gastric cancer (n = 7). They were given one of the following anaesthetics in combination with nitrous oxide in oxygen: epidural bupivacaine, enflurane, pentazocine, ketamine, halothane, epidural bupivacaine and enflurane. Regardless of type of anaesthesia, T3 decreased significantly during and after surgery. T4 and rT3 increased markedly when either enflurane or halothane was given but not with the other anaesthetic agents; they then decreased toward pre-surgical levels after surgery. There was no correlation between changes in rT3 and those in cortisol or free fatty acids. TSH fluctuated little. These results show that the increases in rT3 and T4 during and soon after surgery are due not to surgical trauma but to inhalational anaesthetics such as enflurane and halothane.