5'AMP-activated protein kinase (AMPK) exist as a heterotrimer comprising a catalytic subunit and regulatory and subunits. The AMPK system is activated under conditions of cellular stress, indicated by an increase in the AMP/ATP ratio, as observed e.g. in muscles during contractile activity. AMPK was originally thought to be activated only by local intracellular mechanisms. However, recently it has become apparent that AMPK in mammals is also regulated by humoral substances, e.g. catecholamines. We studied whether humoral factors released during exercise regulate AMPK activity in contracting and resting muscles as well as in abdominal subcutaneous adipose tissue in humans. In resting leg muscle and adipose tissue the AMPK activity was not up regulated by humoral factors during one-legged knee extensor exercise even when arm cranking exercise, inducing a ~20 fold increase in plasma catecholamine level, was added simultaneously. In exercising leg muscle the AMPK activity was increased by one-legged knee extensor exercise eliciting a whole body respiratory load of only 30% VO2 peak but was not further increased by adding arm cranking exercise. In conclusion, during exercise with combined leg kicking and arm cranking, the AMPK activity in human skeletal muscle is restricted to contracting muscle without influence of marked increased catecholamine levels. Also, with this type of exercise the catecholamines or other humoral factors do not seem to be physiological regulators of AMPK in the subcutaneous adipose tissue. Keywords: AMPK, acetyl CoA-carboxylase, ACC, catecholamines