The effects of insulin on energy expenditure are a matter of dispute. Various authors have reported increases or decreases. Irrespective of their nature, it is not clear whether the effects of insulin on energy expenditure are secondary to insulin-induced hypoglycemia or changes in motor activity. The present study investigated the acute effects of insulin on energy expenditure, energy substrate utilisation, motor activity and blood glucose levels. Four U/kg of fast acting insulin had no effect on any of the metabolic or motor activity measures in spite of producing a 30% reduction in blood glucose levels. In contrast, 8 U/kg of insulin increased energy expenditure and respiratory quotient, with the latter effect indicating increased reliance on carbohydrates as a source of energy. This dose reduced blood glucose levels by 68%, yet had no significant effect on motor activity. Insulin, therefore, enhances thermogenesis and carbohydrate utilisation in a manner that can be dissociated from any effect on motor activity. These effects occur at a high dose and they are not counteracted by even massive hypoglycemia. It, therefore, appears that in terms of energy expenditure insulin may be characterised as catabolic, whereas in terms of substrate utilisation it may be characterised as anabolic.