Intense exercise of short duration is heavily dependent on energy from anaerobic sources, and subjects successful in anaerobic types of sports may therefore have a larger anaerobic capacity and be able to release energy at a higher rate. Performances in these kinds of sports are improved by training, suggesting that the anaerobic capacity is trainable. The purpose of this investigation was to study the effect of training on anaerobic capacity. We therefore determined the anaerobic capacity, expressed as the maximal accumulated O2 deficit during treadmill running, of untrained, endurance-trained, and sprint-trained young men. In addition, seven women and five men trained for 6 wk, and their anaerobic capacity was compared before and after the training period. There was no difference in anaerobic capacity between the untrained and endurance-trained subjects, whereas the sprinters' anaerobic capacity was 30% larger (P less than 0.001). The women's anaerobic capacity was 17% less than the men's (P = 0.03). Six weeks of training increased the anaerobic capacity by 10%. We conclude that the anaerobic capacity varies significantly between subjects and that it can be improved within 6 wk. Moreover, there was a close relationship between a high anaerobic capacity and a high peak rate of anaerobic energy release.