The diameters of epididymal fat cells of 12–17-day-old obese and normal littermates were compared following operative removal of the epididymal fat body. The animals were kept alive and checked for obesity at an age of 6 wk. Fat cells of the genetically obese mice began their fast growth between day 12 and day 14. At this age, it is possible to identify on the basis of fat-cell diameters three classes, representing+/+, ob/+, and ob ob, respectively, which shows that the ob allel is incompletely dominant. Measurements of the plasma-insulin concentration revealed that hyperinsulinemia, which is characteristic for the obese hyperglycemic syndrome, is not manifested before the beginning of the fourth week. Since hyperinsulinemia becomes evident at a later stage in development than the rapid increase in growth rate of the fat cells, it seems that the insulin resistance and the hyperinsulinemia cannot be the primary cause of the enlargement of the epididymal adipocytes.