Four human subjects were rendered hyperthermic and hypothermic by immersion in warm and cool water, at 02.00, 08.00, 14.00 and 20.00 hr. Bath and oesophageal temperatures and pulse rate were recorded. Temperature preference was determined by operant behaviour and vote. The core temperature set-point for behavioural thermoregulation was estimated from the behavioural results. 2. The results are in accord with those of previous studies of the nyethemeral cycling of autonomic responsiveness to heat and cold with a heating up phase before noon and a cooling down phase during the early night. 3. Subjective sensations and behavioural responses were also found to follow a nycthemeral cycle with a minimum before noon and a maximum at 20.00 hr. 4. The core temperature set point was 0-7 degrees C higher after noon than before noon with a small phase advance from resting core temperature. This result suggests that the nycthemeral cyclic change in body temperature is due to a nycthemeral cyclic change in the set-point near to which body temperature is kept by both autonomic and behavioural thermoregulatory responses.
