ABSTRACTDaily fluctuations in serum enzyme activities during distance running in 3 well-conditioned males were measured for 8 consecutive days. During the study the exercise regimen comprised 8 mile runs on days 1, 2 and 5; a 10.5 mile run on day 3; a 15 mile run on day 4 and resting on days 6–8. Heart rate, rectal temperature, hematocrit, plasma hemoglobin, creatine phosphokinase (CPK, glutamic-oxaloacetic and glutamic-pyruvic transaminases, lactate dehydrogenase (LDH), adenylate kinase (AK), and lactate and pyruvate were measured before and after exercise and during the resting days. Significant increases occurred in heart rate and rectal temperature with each run but were unrelated to enzyme levels. Pre-exercise CPK levels, 49±1 mIU/ml on day 1, rose progressively to 123± 25 mIU/ml on clay 5, and returned to initial control levels by day 8. Post-exercise CPK levels were significantly greater than pre-exercise levels on each running day hut were unrelated to the severity of the exercise. LDH and AK levels did not significantly change with the exercise stress, but pre-exercise AK levels in these trained males were higher than values in non-trained males (10–20 vs. 0–5 mIU/ml) (3). Post-exercise enzyme levels appeared unrelated to final heart rate, rectal temperature or plasma hemoglobin. These results suggest that (1) CPK, unlike the other enzymes studied, is a sensitive index of exercise stress in well-conditioned runners and (2) elevated CPK and AK levels in such runners represent physiologic responses. Appropriate caution should be used in making clinical judgements from these enzyme assays in trained individuals.