The purpose of this study was to investigate the potentially important role leptin may play during training monitoring in athletes. Twelve highly trained male rowers underwent a 3-week period of maximally increased training stress followed by a 2-week tapering period. Fasting blood was sampled after a rest day. Subjects also performed a maximal 2000-m rowing ergometer test before and after 3 weeks of heavy training, and after 2 weeks of tapering. Blood samples were obtained before, immediately after and after 30 min of recovery. Leptin concentrations were measured in duplicate by radioimmunoassay. Mean training time was about 100% higher during the heavy training period (17.5 h x week(-1)) compared to the tapering period (8.9 h x week(-1)). The 3-week heavy training period induced a significant reduction ( P<0.05) in the fasting leptin concentration [from 2.5 (0.4) to 1.5 (0.4) ng x ml(-1)]. Fasting plasma leptin was significantly increased by the end of the 2-week tapering period [2.0 (0.4) ng x ml(-1)] but remained significantly lower compared to the pretraining value. Leptin levels were also significantly decreased only after the 2000-m rowing ergometer test performed at the end of the heavy training period. No differences in leptin concentrations were observed after other performance tests compared to their respective baseline values. In addition, fasting leptin concentration was significantly related to the weekly training time ( r=-0.45; P=0.006). In conclusion, it appears that leptin is sensitive to the rapid and pronounced changes in training volume. A greater training time is associated with a lower leptin concentration in highly trained male rowers. It is suggested that it may be possible to direct typical rowing training by monitoring leptin status.