To obtain further information on the regulation of lipolysis in vivo, the effect of increasing sympathetic nerve activity via lower body negative pressure (LBNP, -20 mm Hg) was studied in 11 healthy human subjects. Subcutaneous and muscle microdialysis as well as blood flow measurements were performed in the postabsorptive state and during an euglycemic hyperinsulinemic clamp. LBNP for 30 min in the postabsorptive phase resulted in an approximately 50% increase (P < 0.005) in the interstitial-arterial concentration difference for glycerol in adipose tissue, whereas no such effect was registered in muscle. Blood flow in adipose tissue and the forearm remained unaltered. During euglycemic hyperinsulinemic conditions (p-insulin 645 +/- 62 pmol/liter), both interstitial adipose tissue and arterial concentrations of glycerol were reduced. LBNP resulted in an increase in interstitial-arterial concentration difference in glycerol similar to that seen in the postabsorptive state (approximately 50%, P < 0.05). Muscle glycerol was not changed by either insulin or LBNP. Glucose infusion rate during the clamp was significantly decreased during LBNP (7.82 +/- 0.88 vs. 8.67 +/- 1.1 ml/kg.min, P < 0.05). We conclude that the sympathetic nervous activation by LBNP results in an increased lipolysis rate in adipose tissue both in the postabsorptive phase and during insulin infusion. On the other hand, muscle glycerol output was not affected by either LBNP or insulin. The data suggest that 1) lipolysis is regulated differently in muscle and adipose tissue, 2) postabsorptive lipolysis is mainly regulated by insulin, and 3) sympathetic nervous activation effectively inhibits the antilipolytic action of insulin by inducing insulin resistance.