Atractyloside-insensitive binding of purine nucleotides is reduced in brown adipose tissue mitochondria of the obese (ob ob) mouse. Exposure of the ob ob mouse to 4° C does not induce the usual increase in binding. Atractyloside-insensitive binding of purine nucleotides is believed to be a measure of the heat-producing proton conductance pathway in brown adipose tissue mitochondria. It is, therefore, suggested that the impaired thermogenesis of the ob ob mouse is due to a defect in this pathway in the mitochondria of the brown adipose tissue, the major thermogenic tissues in rodents. The greater metabolic efficiency which would result from a reduced operation of this pathway might be the basis for the obesity in the ob ob mouse.