Abstract.Fat cells of different sizes were isolated from the same sample of adipose tissue by collagenase treatment and fractionation by flotation in an isoosmolar medium. Protein, phospholipid and cholesterol increased with increasing fat cell size. Cholesterol correlated most closely with fat cell volume but phospholipid apparently with surface. Most of fat cell cholesterol was found in the triglyceride droplet, and only a smaller part in a particulate fraction from fat cells. Incorporation of labelled glucose into triglyceride increased with fat cell size, apparently in proportion to fat cell surface rather than diameter or volume. This was the case also with incorporation into fatty acids. The association between fat cell size and metabolism was present also in alloxan diabetic rats. Short term insulin deficiency thus does not abolish the increase of metabolic activity with fat cell size. Analyses of activities of selected glycolytic enzymes and of fatty acid synthesis from acetate in cell‐free systems showed a similar dependence on fat cell size, demonstrating that isotope dilution phenomena in large and small fat cells are probably not responsible. — Lipolytic activity in the basal state and after epinephrine stimulation was increased in large fat cells, which also reesterified more fatty acids than small fat cells when calculated according to the balance method. — It was concluded that larger fat cells are metabolically more active than smaller fat cells. The increase of glyceride‐glycerol labelling and of glycerol release from larger fat cells suggests an increased triglyceride turn‐over in these fat cells. Large fat cells might thus be considered as an active metabolic sub‐compartment of adi