A reduction of postprandial thermogenesis has been described in obesity; insulin resistance and/or decreased sympathetic nervous system activity seem to play the major role in its pathogenesis. On the other hand, a normal energy expenditure during exercise has been reported. At present, the response and the role of catecholamines in energy metabolism during exercise in obesity have not been well clarified yet. The aim of this work was to study the metabolic and hormonal changes caused by intense exercise in obesity. Nine obese subjects and ten normal weight controls were submitted to exhaustive exercise on a cycloergometer. Blood glucose, free fatty acids (FFA), glycerol, lactate, beta-OH-butyrate, insulin, glucagon, plasma growth hormone (HGH), catecholamine plasma levels were assayed before and at the end of exercise, and after a recovery period. The energy cost of exercise was evaluated by indirect calorimetry. In our experiment muscular exercise did not provoke any change in blood glucose and FFA plasma levels in either of our groups. In the obese subjects the insulin plasma levels were higher than in the controls. Glucagon plasma levels did not change. The exercise responses of norepinephrine (NE) (4.28 +/- 0.74 vs 8.81 +/- 1.35 nmol/l; P < 0.01), epinephrine (E) (234.21 +/- 64.18 vs 560.51 +/- 83.38 pmol/l; P < 0.01) and plasma growth hormone (HGH) (134.84 +/- 58.97 vs 825.92 +/- 195.25 pmol/l; P < 0.01) were significantly lower in obese subjects. At the end of exercise, the thermic effect of exercise did not differ between obese and control subjects (0.335 +/- 0.038 vs 0.425 +/- 0.040 kJ/min x kg fat-free mass. Our findings indicate that an impaired counterregulatory hormone response to exercise exists in obese subjects. The thermic effect of exercise does not seem to be affected by either the reduced catecholamine response nor insulin resistance.