We investigated the effect of increased plasma adrenaline on hormone-sensitive lipase (HSL) activity and extracellular regulated kinase (ERK) 1/2 phosphorylation during exercise. Seven untrained men rested for 20 min and exercised for 10 min at 60 % peak pulmonary oxygen uptake on three occasions: with adrenaline infusion throughout rest and exercise (ADR), with no adrenaline infusion (CON) and with adrenaline infusion commencing after 3 min of exercise (EX+ADR). Muscle samples were obtained at rest before (Pre, -20 min) and after (0 min) infusion, and at 3 and 10 min of cycling. Exogenous adrenaline infusion increased (P < 0.05) plasma adrenaline at rest during ADR, which resulted in greater HSL activity (Pre, 2.14 +/- 0.10 mmol min-1 (kg dry matter (dm))-1; 0 min, 2.74 +/- 0.20 mmol min-1 (kg dm)-1). Subsequent exercise had no effect on HSL activity. During exercise in CON, HSL activity was increased (P < 0.05) above rest at 3 min but was not increased further by 10 min. The infusion of exogenous adrenaline at 3 min of exercise in EX+ADR resulted in a marked elevation in plasma adrenaline levels (3 min, 0.57 +/- 0.12 nM; 10 min, 10.08 +/- 0.84 nM) and increased HSL activity by 25 %. HSL activity at 10 min was greater (P < 0.05) in EX+ADR compared with CON. There were no changes between trials in the plasma concentrations of insulin and free fatty acids (FFA) and the muscle contents of free AMP, all putative regulators of HSL activity. ERK1/2 phosphorylation increased at 3 min in CON and EX+ADR. Because HSL activity did not increase during exercise when adrenaline was infused prior to exercise (ADR) and because HSL activity increased when adrenaline was infused during exercise (EX+ADR), we conclude that (1) high adrenaline levels can stimulate HSL activity regardless of the metabolic milieu and (2) large increases in adrenaline during exercise, independent of changes in other putative regulators, are able to further stimulate the contraction-induced increase in HSL activity. The results also demonstrate that increased ERK 1/2 phosphorylation coincides with elevated HSL activity, indicating that ERK 1/2 may mediate the contraction-induced increase in HSL activity early in exercise.