BACKGROUND: Energy restriction and physical activity are major components of weight reduction programs. Energy restriction is known to affect the sympathetic nervous system activity and to reduce several components of energy expenditure, including the stimulation of energy expenditure elicited by mental stress. The effect of physical activity on this parameter remains unknown. METHODS: This study was designed to assess the effect of physical training on the stimulation of energy expenditure during mental stress. Seven obese women (age 33 +/- 3 years, BMI 34.9 +/- 1.4 kg/m(2)) were studied before and after a 6 week program of aquagym (2 sessions of 50 min/week). Energy expenditure was measured by means of indirect calorimetry under hyperinsulinemic conditions before and during a 30 min mental stress. O(2) max was measured before and after physical training by means of the test of Balke. RESULTS: O(2) max was 23.4 ml/kg/min before physical training, and increased to 26.1 ml/kg/min at the completion of the training program (p<0.05). Body weight, resting energy expenditure and insulin-mediated glucose disposal [oxidative and non oxidative] were not modified after training. Mental stress increased energy expenditure by 12.3% before physical training (p<0.002 vs resting conditions) and by 12.6% after physical training (p<0.003 vs resting conditions, NS vs before physical training). CONCLUSION: Physical training per se does not alter the stimulation of energy expenditure induced by mental stress.