The development of obesity in preweaning obob mice.

1. The body compositions of obob and lean (ob+ and ++) mice at 10, 12, 17 and 28 d of age were investigated using a 'cold stress' test to identify the two groups. 2. At each of these ages the obob mice were found to contain significantly more fat than the lean. At 10 d 20% more fat was present and by 17 d the increase was 72%. The obob mice at 28 d contained nearly three times as much fat as the lean. 3. Carcass energy was significantly higher in obob mice at all ages investigated. 4. Other changes in body composition found in the 28 d obob mice, i.e. a reduction in total carcass nitrogen and water content, were already established in the 17-d-old mice but differences at 10 and 12 d were not apparent. 5. The livers of obob mice were significantly heavier than those from lean control mice at 28 d but no differences were detected at the earlier ages. 6. The results are discussed with reference to the early origin of obesity in obob mice.