Overtraining (OT) is a complex and multifactorial sport phenomenon, and there is no independent marker that can diagnose OT. Interestingly, some symptoms of OT are related to beta-endorphin (beta-end(1-31)) effects. Some of its effects, such as analgesia, increasing lactate tolerance, and exercise-induced euphoria, are important for training. These effects can be reverted by detraining or OT, which may cause decrease in performance, reduced load tolerance, and depression. The main stimulus for beta-end(1-31) secretion is to exercise because its secretion is volume/intensity dependent for both aerobic and anaerobic exercise. Excess training, however, may reduce beta-end(1-31) concentrations, thus altering its beneficial effects. Therefore, beta-end(1-31) could be used as an additional OT marker, mainly because its effects are strongly related to OT symptoms.