Six albino rats were exposed to low ambient temperature (4[degree]C) and trained to depress a response bar to get external heat from an infrared lamp. The lamp remained on as long as the bar was depressed. The total time of bar depression over a one hour test session increased as a function of food deprivation periods of 24, 48 and 72 hours. This increase in amount of external heat requirement appears to directly reflect the reduced energy supply of the animal.