BACKGROUND: The blood flow capacity in subcutaneous adipose tissue in humans remains largely unknown, and therefore the aim of this study was to determine the physiological range of blood flow in this tissue. METHODS AND RESULTS: The subcutaneous adipose tissue blood flow (ATBF) was measured in 9 healthy young men by positron emission tomography using radiowater tracer. Subcutaneous ATBF was determined in regions adjacent to knee extensors at rest and during dynamic knee extensor exercise, and with 2 physiological perturbations: while breathing moderate systemic hypoxic air (14% O2) at rest and during exercise, and during intra-femoral artery infusion of high-dose adenosine infusion. ATBF was 1.3+/-0.6ml.100g(-1).min(-1) at rest and increased with exercise (8.0+/-3.0ml.100g(-1).min(-1), P<0.001) and adenosine infusion (10.5+/-4.9ml.100g(-1).min(-1), P=0.001), but not when breathing moderate systemic hypoxic air (1.5+/-0.4ml.100g(-1).min(-1)). ATBF was similar during exercise and adenosine infusion, but vascular conductance was lower during adenosine infusion. Finally, ATBF during exercise in moderate systemic hypoxia was reduced (6.3+/-2.2ml.100g(-1).min(-1)) compared to normoxic exercise (P=0.004). CONCLUSIONS: The vasodilatation capacity of human subcutaneous adipose blood flow appears to be comparable to, or even higher, than that induced by moderate intensity exercise. Furthermore, the reduced blood flow response in subcutaneous adipose tissue during systemic hypoxia is likely to contribute, in part, to the redistribution of blood flow to exercising muscle in a condition of reduced oxygen availability