Nine obese women without hyperinsulinemia fasted 22 +/- 2 d under controlled conditions. Blood was drawn weekly and lipids, lipoproteins, and insulin were determined. In five patients an open fat biopsy was performed before and after fasting and the incorporation of free fatty acids (FFA) into total lipids, diglycerides (DG), and triglycerides (TG) was measured. Glycerol release into the medium was also determined. During fasting triglycerides, total cholesterol, LDL-cholesterol, HDL-cholesterol, glucose, and insulin fell considerably. The uptake and esterification of FFA decreased under both basal and stimulated (norepinephrine) conditions (-11 percent to -23 percent). Basal lipolysis increased slightly by 11 percent. Results indicate that fasting-induced changes in adipose tissue metabolism do not account for the decrease in serum triglycerides. The diminished triglyceride synthesis and the enhanced breakdown of triglycerides in adipose tissue do, however, explain the reduction in adipose tissue mass during fasting.