This study determined the effects of different conditioning levels on exertional rhabdomyolysis after severe exercise. 15 well-conditioned men were randomly divided into three conditioning groups, i.e. 0, 4 and 8 miles on alternate days for 1 month. They were physically stressed by running 14 miles at 8 min/mile after 1 month of training. The effects of the different levels of conditioning on exertional rhabdomyolysis after the test run were determined by assaying serum samples for creatine kinase (CK) and myoglobin levels. Serum CK levels increased 98-294% above pre-conditioning control values after the 14-mile test run while serum myoglobin levels increased 52-405%. The most marked increases occurred in those subjects who trained less than 8 miles on alternate days. These results suggest that these quantitative indices are useful in determining the amount of conditioning necessary to prevent significant rhabdomyolysis after severe exercise.