To investigate the antilipolytic effect of insulin in skeletal muscle and adipose tissue in vivo, the rates of glycerol release from the two tissues were compared in 10 nonobese women during a two-step euglycemic hyperinsulinemic clamp. Tissue interstitial glycerol levels were determined by microdialysis, and tissue blood flow was assessed with the ¹³³Xe clearance technique. Absolute rates of glycerol release were estimated according to Fick's principle. In both adipose tissue and muscle, glycerol levels decreased significantly already during the low insulin infusion rate. The fractional release of glycerol (difference between interstitial glycerol and arterialized venous plasma glycerol) was reduced by more than one-half in adipose tissue (P < 0.0001) in response to insulin, whereas it remained unaltered in skeletal muscle. Muscle blood flow rates increased by 60% (P < 0.02) during insulin infusion; in adipose tissue, blood flow rates did not change significantly in response to insulin. The basal rate of glycerol release from skeletal muscle amounted to ~15% of that from adipose tissue. After insulin infusion, the rate of adipose tissue glycerol release was markedly suppressed, whereas in skeletal muscle the rate of glycerol mobilization did not change significantly in response to insulin. It is concluded that insulin does not inhibit the rate of lipolysis in skeletal muscle of nonobese women.