The relationship between food intake and oxygen consumption was studied in a group of 60 rats acclimated at environmental temperatures of either 30 or 10 °C. Three separate experiments were performed. In the first, 28 rats were divided into two groups: control, which received 20 and 32 g of food/day at 30 and 10 °C, respectively, from 0800 to 1700 and experimental, which received 10 and 25 g of food/day at 30, and 10 °C, respectively. The experimental period lasted 6 weeks. Oxygen consumption was measured weekly at environmental temperatures of 5, 15, 25, 30, and 35 °C. In the second experiment, 16 rats were subjected to the same food intake as the animals in the first experiment. After 1 week, their oxygen consumption was measured at 25 °C over a period of 24 h. The third experiment was carried out with 16 other rats in which the control groups received the same amount of food as in the first experiment, and the experimental groups were fed 6 and 11 g/day at 30 and 10 °C, respectively, during 1 week. In the first experiment, no changes in oxygen consumption (per kilogram 0.67) were apparent in the experimental rats during 6 weeks. However, after 1 week on severe food restriction a significant decrease in oxygen consumption (per kilogram 0.67) was observed. A long-lasting thermic effect of food was observed in control rats from the second experiment and a rapid effect was apparent in restricted rats. The results seem to indicate that in the rat, although the oxygen consumption does depend on the level of food intake, it is necessary to have a higher level of restriction to make it apparent when compared with larger species of animals. It is also suggested that energy conservation mechanisms did not interfere with cold acclimation.