The stimulus for the release of Hsp72 during exercise in humans is currently unclear. Recent evidence in an animal model is suggestive of an involvement of catecholamines. The present study therefore investigated the effect of caffeine supplementation, a known stimulator of sympathetic activity, on the extracellular (e)Hsp72 response to prolonged exercise. Ten healthy male endurance trained cyclists were recruited (age: 21 ± 1 years, VO₂max 61.1 ± 1.7 ml/kg/min, mean ± SEM). Each subject was randomly assigned to ingest either 6mg/kg body mass of caffeine (CAFF) or placebo (PLA) 60 minutes prior to one of two 90 minute bouts of cycling at 74 ± 1 % VO₂max. Trials were performed at least 7 days apart in a counterbalanced design. Venous blood samples were collected by venepuncture at pre-treatment, pre exercise, post exercise and 1h post exercise. Serum caffeine and plasma catecholamines were determined using a spectrophotometric assay and high performance liquid chromatography respectively. Plasma Hsp72 and cortisol were determined by ELISA. Serum caffeine concentrations were significantly increased throughout CAFF while no increases were detected in PLA. Caffeine supplementation and exercise was associated with a greater eHsp72 response than exercise alone (Post exercise CAFF 8.6± 1.3 ng/ml; PLA 5.9± 0.9 ng/ml). This greater eHsp72 response was associated with a greater epinephrine response to exercise in CAFF. There was a significant increase in norepinephrine and cortisol, with no intertrial differences. The present data suggests that, in humans, catecholamines may be an important mediator of the exercise-induced increase in eHsp72 concentration.