This paper presents data to demonstrate a loosening in the coupling of phosphorylation to respiration in brown adipose tissue mitochondria during cold exposure. This loosening of coupling was accompanied by a significant increase in the respiratory rate. Moreover, the mitochondrial energy conserving mechanism has been shown to recouple when cold exposed animals are returned to thermoneutral surroundings. The degree of coupling of phosphorylation to respiration has been investigated in terms of the P/O ratio and of the respiratory control index. With brown adipose tissue mitochondria, both of these relationships have to be studied independently in order to obtain data relevant to the problem of variations in the energy conserving mechanism since they are differently affected by the in vitro conditions used. Our findings indicate that the non-shivering thermogenesis of brown adipose tissue is physiologically regulated and perhaps in a more sophisticated way than by simple on-off control. Guinea-pigs have been used in the experiments.