The relationship between maximal aerobic capacity [Formula: see text] and blood enzyme level changes during long marching was studied in two groups of healthy untrained volunteers aged 18–19, who covered 110 km in 2 days. Group A (nine subjects) walked at a uniform rate of 6 km/h, while group B (17 subjects) walked at either 4, 5, or 6 km/h, according to whether their [Formula: see text] was, respectively, low, moderate, or high. In group A there were striking elevations in the activities of muscle enzymes, e.g. creatine phosphokinase, glutamic–oxaloacetic transaminase, and aldolase, at the end of the march, while the elevations in group B were moderate at most. A classification of the subjects in group A according to their maximal oxygen consumption showed that the highest enzyme elevation appeared in the subjects with the lowest [Formula: see text] and moderate elevation in those with high [Formula: see text]. In group B, whose subjects were prefitted to exercise intensity according to each subject's [Formula: see text], no enzymatic differences between the subgroups appeared. These findings suggest that enzyme elevations are primarily related to the intensity of the effort with respect to the [Formula: see text] of the individual and not to its duration. Adjusting work intensity to the level of [Formula: see text] should minimize muscle enzyme leakage and muscular changes.