This study examines a new hypothesis whereby heat production from brown fat in response to eating may serve as a feedback signal for satiety. To test this hypothesis, in vitro respiration rate of brown adipose tissue (BAT) was determined in relation to the voluntary caloric intake of the preceding test meal. This relationship was examined as a function of meal composition and of obesity. It was found that in rats fed a high fat diet, as well as in two types of obese rats (VMH and Zucker), respiration rate per 100 mg tissue was significantly reduced, and energy intake of the preceding test meal increased compared to rats receiving a low fat diet or to respective lean rats. These data lend support to a brown fat mediated thermostatic hypothesis for the control of food intake.