Acute effect of aerobic exercise on serum leptin concentration has been subject of inconsistent results. A recent review concluded that, in order to modify serum leptin concentration a single session of aerobic exercise must either be at least longer that 60 min. or consume more than 800 Kcal (Bouassida et al 2008). The present study was designed to assess the effects of 50 min. of aerobic exercise with a caloric expenditure of ~540 Kcal. on short-term serum leptin levels in fasting subjects and following carbohydrate (CHO) intake. Ten healthy and physically-active males were randomly assigned to four testing conditions: a) FR: fasting-rest b) FE: fasting-exercise; c) BR: breakfast-rest; d) BE: breakfast-exercise. Blood samples were collected every 30 minutes during the 8-hour testing conditions for analysis of plasma leptin, insulin and blood glucose. Data were analyzed by two way repeated measures ANOVA and simple effects follow up. During all testing conditions, significant reductions in the serum leptin levels were observed (p < 0.05). Our data show that: 1. Aerobic exercise did not induce direct alterations of serum leptin levels neither in fasting nor in fed subjects since no significant differences were detected when comparing BR vs. BE and FR vs. FE conditions. 2. Exercise diminished postprandial glucose and insulin concentrations: the BE condition elicited increases in glucose and insulin levels, but these were significantly lower than for the BR condition (p < 0.05). 3. A single meal (439 Kcal.) with a high CHO content induced an increase of serum leptin concentration: starting at 14:00 hours (five hours after CHO intake), serum leptin in the BR condition was significantly higher than in the FR condition (p < 0.05). 4. Aerobic exercise abolished the observed CHO effect on serum leptin levels since no significant differences were found when comparing FR and BE conditions. Thus, 50 min of aerobic exercise with a caloric expenditure of ~540 Kcal. is sufficient to modify leptin response to feeding.