It is believed that brain derived neurotrophic factor (BDNF) plays an important role in neuronal growth, transmission, modulation and plasticity. Single bout of exercise can increase plasma BDNF concentration [BDNF](p) in humans. It was recently reported however, that elevated [BDNF](p) positively correlated with risk factors for metabolic syndrome and type 2 diabetes mellitus in middle age group of subjects. On the other hand it is well established that endurance training decreases the risk of diabetes and development of metabolic syndrome. In the present study we have examined the effect of 5 weeks of moderate intensity endurance training on the basal and the exercise induced changes in [BDNF](p) in humans. Thirteen young, healthy and physically active men (mean +/- S.E: age 22.7 +/- 0.5 yr, body height 180.2 +/- 1.7 cm, body weight 77.0 +/- 2.5 kg, V(O2max) 45.29 +/- 0.93 ml x kg-1 x min(-1)) performed a five week endurance cycling training program, composed mainly of moderate intensity bouts. Before training [BDNF]p at rest have amounted to 10.3 +/- 1.4 pg x ml(-1). No effect of a single maximal incremental cycling up to V(O2max) on its concentration was found (10.9 +/- 2.3 pg x ml(-1), P=0.74). The training resulted in a significant (P=0.01) increase in [BDNF]p at rest to 16.8 +/- 2.1 pg x ml(-1), as well as in significant (P=0.0002) exercise induced increase in the [BDNF](p) (10.9 +/- 2.3 pg x ml(-1) before training vs. 68.4 +/- 16.0 pg x ml(-1) after training). The training induced increase in resting [BDNF](p) was accompanied by a slight decrease in insulin resistance (P=0.25), calculated using the homeostatic model assessment version 2 (HOMA2-IR), amounting to 1.40 +/- 0.13 before and 1.15 +/- 0.13 after the training. Moreover, we have found that the basal [BDNF](p) in athletes (n=16) was significantly higher than in untrained subjects (n=13) (29.5 +/- 9.5 pg x ml(-1) vs. 10.3 +/- 1.4 pg x ml(-1), P=0.013). We have concluded that endurance training of moderate intensity increases both basal as well as the end-exercise [BDNF](p) in young healthy men. This adaptive response, contrariwise to the recent findings in patients with metabolic disorders, was accompanied by a slight decrease in insulin resistance.