YOUNG genetically obese (ob/ob) mice develop obesity even when pair-fed to their lean littermates1–3. This is due to an unusually high metabolic efficiency, which we have suggested results from a reduction in the “maintenance increment” to the basal metabolic rate (BMR)4. Factors involved include the thermic effect of food and the thermogenic response to an environmental temperature below thermoneutrality. Part of the evidence supporting this suggestion comes from the observation that mature ob/ob mice, and at least two other strains of genetically obese rodent, are unable to maintain their body temperature when exposed to cold5–7. At 4 °C both the ob/ob mouse and the Zucker (fa/fa) rat rapidly die of hypothermia5,7. In the mouse, thermogenesis rather than heat conservation has been established as the cause of the thermoregulatory defect5,7. We now report that a defective response to cold is apparent in preweanling mice bearing the ob/ob genotype about 2 weeks before they can be identified as obese by visual inspection. From this a test based on the response to 4 °C has been developed for the identification of ob/ob mice at 17 d old.