Adenosine is a widely used pharmacological agent to induce a 'high flow' control condition to study the mechanisms of exercise hyperemia, but it is not known how well adenosine infusion depicts exercise-induced hyperemia especially in terms of blood flow distribution at the capillary level in human muscle. Additionally, it remains to be determined what proportion of adenosine-induced flow elevation is specifically directed to muscle only. In the present study we measured thigh muscle capillary nutritive blood flow in nine healthy young men using positron emission tomography at rest and during femoral artery infusion of adenosine (1 mg * min(-1) * litre thigh volume(-1)), which has previously been shown to induce maximal whole thigh blood flow of ~8 L/min. This response was compared to the blood flow induced by moderate-high intensity one-leg dynamic knee extension exercise. Adenosine increased muscle blood flow on average to 40 +/- 7 ml. min(-1) per 100g(-1) of muscle and an aggregate value of 2.3 +/- 0.6 L * min(-1) for the whole thigh musculature. Adenosine also induced a substantial change in blood flow distribution within individuals. Muscle blood flow during adenosine infusion was comparable to blood flow in moderately-high intensity exercise (36 +/- 9 ml. min(-1) per 100g(-1)), but flow heterogeneity was significantly higher during adenosine infusion than during voluntary exercise. In conclusion, a substantial part of the flow increase in the whole limb blood flow induced by a high dose adenosine infusion is conducted through physiological non-nutritive shunt in muscle and/or also through tissues of the limb other than muscle. Additionally, intra-arterial adenosine infusion does not mimic exercise hyperemia especially in terms of muscle capillary flow heterogeneity, while the often observed exercise-induced changes in capillary blood flow heterogeneity likely reflect true changes in nutritive flow linked to muscle fibre and vascular unit recruitment. Key words: adenosine, exercise, muscle, blood flow.