AIM To evaluate the effect of a short-term sibutramine intake on energy expenditure and on physiological responses during rest, submaximal exercise and maximal exercise in obese women. METHODS Fifteen healthy obese female volunteers were randomly assigned to either a placebo (n = 6 - control) or sibutramine group (n = 9 - experimental) under double-blind conditions. Each subject was tested under resting conditions and then performed submaximal and maximal exercise tests. Subjects were retested under identical conditions following a 5-day administration of either sibutramine 10 mg x day(-1) or a placebo. All measurements (i.e. weight, body composition, resting energy expenditure, heart rate, respiratory exchange ratio, ventilation, oxygen consumption, carbon dioxide production, systolic blood pressure, diastolic blood pressure and blood lactate concentration) were analyzed using a 2-way ANOVA with repeated measures. RESULTS Only heart rate during submaximal exercise was significantly different (higher) following short-term sibutramine administration when compared to placebo (p < 0.05). CONCLUSIONS These data suggest that a recommended daily dose of 10 mg sibutramine does not affect energy expenditure or other selected cardiopulmonary responses during rest, submaximal exercise or maximal exercise. It is, therefore, suggested that weight loss during sibutramine treatment in humans is achieved mostly via a decrease in energy intake rather than through an increase in energy expenditure and thermogenesis.