The superficial and deep body temperatures of 23 patients were studied during prolonged anaesthesia for microscopic surgery of the fallopian tubes. The patients were divided randomly into three groups, receiving either halothane 0.5%, halothane 1% or low-dose fentanyl as a supplement to nitrous oxide, oxygen and myoneural block. Other variables were kept constant, and the theatre temperature was maintained at 24 °C. Temperature changes were unrelated to both the type of anaesthesia and the percentage of subcutaneous fat to body weight. Halothane 1 % decreased the rate of heat loss in the 3rd hour. Large heat losses occurred on transfer to the recovery room, where the total heat produced increased rapidly and was unrelated to shivering.