Box turtles were implanted with thermodes astride the preoptic tissue of the brainstem. The rate of evaporative water loss could be transiently increased by heating the rostral brainstem. Heating tissue in the anterior hypothalamus affected evaporative water loss only a high ambient temperatures. The magnitude of the response was proportional both to the change in hypothalamic temperature and to the ambient temperature with which the turtle was in equilibrium. The major fuction of a high rate of evaporative water loss in turtles is probably to protect the brain from overheating during thermal stress.