Obesity currently qualifies as a worldwide health epidemic. With the cloning of mouse ob gene and its receptor leptin was discovered. Leptin is expressed and secreted primarily by adipose tissue and is highly correlated to body fat mass. Nevertheless many factors can regulate leptin synthesis and expression, such as fasting, sympathetic activity, insulin, exercise and changes in energy balance. Endurance training effects on leptin are still contradictory. PURPOSE: To assess the effects of swimming training on leptin levels, the contribution of leptin expression in visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) and the association between leptin, insulin, body composition, basal metabolic rate and energy intake in rats. METHODS: Male Wistar rats were separated in two groups: trained and sedentary control. The exercised-trained group was submitted to 9 weeks of endurance swimming training. Body weight and food intake were recorded weekly. Oral glucose tolerance tests were performed before and during the last week of exercise training. Furthermore, basal metabolic rate, leptin and insulin levels, body fat mass and leptin expression in SAT and VAT were determined after training. RESUITS: Energy intake and basal energy expenditure were not different between groups. Trained rats had a 10% lower final body weight, 36% lower body fat mass, 55% lower insulin levels and improved glycemic response (p<0,05 for all variables). Although trained animals had lower leptin levels, no difference in leptin expression in either fat depot was found between groups. According to stepwise multiple regression analysis lower leptin levels in trained rats were due primarily to their lower body fat mass. Nonetheless, after adjustment for body fat mass leptin levels were still 28,6% (p<0,05) lower in exercised rats. CONCLUSIONS: We have found that 9 weeks of swimming training does not affect leptin expression, while leading to a decrease in leptin levels independently of changes in total body fat mass. Theses results suggest that there might be other factors rather than fat mass responsible for the decreased leptin levels found in trained animals, being insulin the primary candidate for such modulation role. Supported by Fapesp (04/03888-3)