The purpose of this study was to clarify the effect of physical activity on the level of serum brain-derived neurotrophic factor (BDNF). The serum BDNF level in trained men who have participated in regular sport activity (n = 12) was compared to that in sedentary subjects (n = 14). The physical activity levels expressed as total energy expenditure, move-related energy expenditure and walking count in the trained were significantly higher than those in the sedentary. The serum BDNF level in the trained men was found to be lower than that in the sedentary (19.54 +/- 4.53 ng/ml vs. 23.63 +/- 2.94 ng/ml, respectively, P < 0.01). The serum BDNF level showed a significant negative correlation with daily total energy expenditure (r = -0.507, P < 0.05), movement-related energy expenditure (r = -0.503, P < 0.05), and walking count (r = -0.480, P < 0.05). These results may suggest that vigorous habitual physical activity decrease the serum BDNF level. (C) 2008 Elsevier Ireland Ltd. All rights reserved.