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1 Possible nitric oxide (NO)-mediated eects on lipolysis were investigated in vivo in human subcutaneous adipose tissue using microdialysis, as well as in vitro on isolated fat cells of non-obese, healthy volunteers. NO donors were added to the ingoing dialysate solvents. 2 Changes in lipolysis and local blood¯ow were investigated by measuring glycerol levels and ethanol ratios, respectively, in the microdialysates. 4 Nitric oxide gas as well as the NO donor, nitroglycerine, reduced glycerol release from isolated adipocytes in vitro. 5 Expression of inducible nitric oxide synthase (iNOS) in human adipose tissue was shown by Western blot analysis. Biologically active NOS was demonstrated by measuring total enzymatic activity. 6 In conclusion, the data demonstrate that inhibition of NO release in subcutaneous adipose tissue results in an increased lipolysis in vivo. These eects, which were also observed in vitro, are independent of local blood¯ow changes. Furthermore, the demonstration of enzymatic NOS activity and the expression of inducible nitric oxide synthase (iNOS) in adipose tissue indicate that locally synthesized NO may play a role in the physiological control of lipolysis in human adipose tissue.