[14C]Acetate incorporation into different lipid fractions was measured as a function of adipocyte size by using the larger and smaller adipocytes derived from Sprague-Dawley rats. In both the larger and smaller adipocytes, [14C]acetate was incorporated into phospholipid, diacylglycerol, free fatty acid and triacylglycerol fractions. Although the rates of lipid formation were significantly higher in the larger adipocytes compared to the smaller ones, the proportions of the various lipids formed from [14C]acetate did not change significantly as a function of cell size. In some experiments, isolated adipocytes derived from obese Zucker rats were fractionated further to isolate an adipocyte preparation which was similar in size to those obtained from lean animals. The matching adipocytes derived from lean and obese animals did not differ significantly with respect to lipid formation from [14C]acetate. These studies suggest that the larger adipocytes are more active in lipogenesis from [14C]acetate than the smaller ones and that the increased capacity of lipogenesis in obese adipose tissue noted previously (Biochem. J., 170, 153-160, 1978) is not an intrinsic property of all the obese adipocytes, but is limited mainly to the larger adipocytes.