The conversion of [14C]-labeled compounds such as acetate, glucose, pyruvate and palmitate into CO2, glyceride-glycerol, glyceride fatty acids and total lipids was monitored in the average and matching adipocyte (with respect to size) preparations from young (6-9 wk) and old (age 56-60 wk) male Sprague-Dawley rats. The average cell size populations from young and old rats were 46 +/- 3 and 83 +/- 11 microns in diameter, respectively. The incorporation of [14C]acetate, pyruvate and glucose into fatty acids was significantly reduced in the adipocytes from older rats, irrespective of their sizes. The production of CO2 and glyceride-glycerol did not change significantly as a function of either cell size or animal age. Palmitate incorporation into lipids was similar in the average cell population derived from old and young rats, but it was considerably lower in the smaller adipocytes (46-50 microns diameter) from old animals. Irrespective of the cell size, triacylglycerol formation from sn-glycerol-3-phosphate was also significantly diminished in the adipocytes from older animals compared to younger ones as evidenced by decreases in activities of several enzymes, including sn-glycerol-3-phosphate acyltransferase, Mg2+-dependent phosphatidate phosphohydrolase and diacylglycerol acyltransferase. However, triacylglycerol formation from monoacylglycerol did not change as a function of either cell size or age. These measurements of the metabolic and enzymic activities provide evidence that the synthesis of fatty acids from various precursors and triacylglycerol formation from sn-glycerol-3-phosphate are significantly reduced in adipocytes from older animals and that such changes occur independently of adipocyte size.