To compare arterial, arterialized venous and venous measurements of plasma catecholamines during hypoglycaemia, six healthy men were studied by an iv infusion of insulin (1034 pmol·kg−1·h−1), Arterial blood glucose was clamped at the baseline level for the first 30 min and then reduced to 3.2 and to 2.5 mmol/l for 20 min subsequently. At each stage, arterial, arterialized venous and venous blood samples were simultaneously taken for analyses of plasma catecholamines. Plasma glucagon significantly declined during hyperinsulinaemic euglycaemia and then increased during hypoglycaemia. Plasma adrenaline and noradrenaline increased significantly during hypoglycaemia. During euglycaemia and mild hypoglycaemia, arterial adrenaline tended to be higher and arterial noradrenaline lower as compared with the corresponding arterialized and venous values. During marked hypoglycaemia, the arterial catecholamines were significantly higher than that of the two venous samples (p<0.05). There was a significant correlation between the arterial and the arterialized as well as venous catecholamine values (p<0.005). We conclude that, during euglycaemia and mild hypoglycaemia, the arterialized adrenaline values are closer to the arterial levels whereas the arterialized noradrenaline concentrations are similar to those measured in venous blood. However, during marked hypoglycaemia, both the arterialized adrenaline and noradrenaline are 20–30% lower than the arterial values, being close to the venous levels.