Muscle glycogen degradation is catalysed by glycogen phosphorylase, which exists in a less active b and more active a form. During exercise, transformation of phosphorylase b to a is regulated by increases in cytosolic Ca 2+ , liberated during muscle contraction, and an adrenaline-mediated increase in 3fi,5fi-cyclic adenosine monophosphate (cAMP), thereby setting an upper limit for muscle glycogenolysis (Howlett et al. 1998). Factors linked to the energy state of the cell (e.g. AMP and IMP) or substrate (glycogen, inorganic phosphate -P i ) then 'fine-tune' the flux rate according to the metabolic demands (Ren & Hultman, 1990;Johnson, 1992).Increased adrenaline often (Jansson et al. 1986;Spriet et al. 1988;Febbraio et al. 1998), but not always (Chesley et al. 1995;Wendling et al. 1996), results in increased muscle glycogen utilisation during exercise in man. The discrepancies in these studies may be related to the infusion of supraphysiological concentrations of adrenaline (Jansson et al. 1986;Spriet et al. 1988) and/or the marked differences in exercise intensity (Chesley et al. 1995;Febbraio et al. 1998). During exercise where the energy state of the cell is challenged (Chesley et al. 1995), factors such as increased P i , and allosteric modulation via increased free AMP (AMP f ) may maximally activate 1. To evaluate the role of adrenaline in regulating carbohydrate metabolism during moderate exercise, 10 moderately trained men completed two 20 min exercise bouts at 58 ± 2 % peak pulmonary oxygen uptake (V O 2 ,peak ). On one occasion saline was infused (CON), and on the other adrenaline was infused intravenously for 5 min prior to and throughout exercise (ADR). ; means ± S.E.M.) and this effect was maintained throughout exercise. Total carbohydrate oxidation increased by 18 % and this effect was due to greater skeletal muscle glycogenolysis (P < 0.05) and pyruvate dehydrogenase (PDH) activation (P < 0.05, treatment effect). Glucose rate of appearance was not different between trials, but the infusion of adrenaline decreased (P < 0.05, treatment effect) skeletal muscle glucose uptake in ADR.3. During exercise muscle glucose 6-phosphate (G-6-P) (P = 0.055, treatment effect) and lactate (P < 0.05) were elevated in ADR compared with CON and no changes were observed for pyruvate, creatine, phosphocreatine, ATP and the calculated free concentrations of ADP and AMP.4. The data demonstrate that elevated plasma adrenaline levels during moderate exercise in untrained men increase skeletal muscle glycogen breakdown and PDH activation, which results in greater carbohydrate oxidation. The greater muscle glycogenolysis appears to be due to increased glycogen phosphorylase transformation whilst the increased PDH activity cannot be readily explained. Finally, the decreased glucose uptake observed during exercise in ADR is likely to be due to the increased intracellular G-6-P and a subsequent decrease in glucose phosphorylation.