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1. Experiments were conducted to compare the abilities of epididymal adipocytes from mice selected for growth (line G) and from unselected mice (line C) to: (a) incorporate glucose, (b) respond to insulin, and (c) mobilize lipids, and to relate these abilities to the food intake of the donors.

2. During the 3-week pre-experimental period, line G mice gained body weight 78% faster and ate 35% more food than line C mice; both lines had similar intakes per unit of metabolic body size.

3. Line G epididymal fat pads weighed 227% more than those of line C and contained adipocytes which were 38% larger; it was estimated that they contained approximately 65% more cells.

4. The basal rate of glucose incorporated into lipids (per unit protein) was highest in line C adipocytes, whereas the basal rates of glucose oxidation to CO₂ and the total glucose incorporation (uptake) were similar for adipocytes from both lines.

5. Insulin (1000 µu./ml.) caused adipocytes from both lines of mice to increase significantly the incorporation of glucose into CO₂ and lipids; the largest elevation occurred when the incubation medium contained 0·1 mg glucose/ml. At this concentration of glucose, the minimum effective dose (MED) of insulin to produce a significant increase in glucose oxidation was similar for both lines. However, the MED of insulin necessary to significantly increase glucose incorporation into lipids and into the sum of CO₂ and lipids was highest in the larger, line G adipocytes. Furthermore, the magnitude of the insulin-induced increase in glucose incorporation was much less for line G than for the line C adipocytes.

6. Epinephrine significantly elevated the rates of NEFA and glycerol release and NEFA re-esterification. The highest rates of NEFA and glycerol release occurred in line C adipocytes, whereas the highest rate of NEFA re-esterification occurred in the line G adipocytes.

7. Glucose had no effect on NEFA release but significantly elevated the rates of glycerol release (in most instances) and NEFA re-esterification.

8. The larger number of adipocytes in line-G adipose tissue allows for more total incorporation of glucose and a greater overall release of glycerol by this tissue. Consequently, a greater reduction of glucose concentration and a larger elevation of glycerol concentration in the blood can occur; either of these could be the feed-back signal which resulted in the larger long-term food intake by the line G mice.