Acute effects of repeated sprinting upon serum creatine kinase (CK), lactic dehydrogenase (LDH), and isozymal activities were studied in five collegiate runners (R_s) and six non-runners (NR_s ). After an intermittent running treadmill test, blood sampling showed three-fold mean increases in CK with no change in LDH in both groups; group differences were insignificant (p>.05). Results suggest (1) intense anaerobic exercise produces moderate enzyme elevations; (2) relatively equivalent exercise intensities are critical to enzyme responses in exercising individuals of varying fitness levels; and (3) exercise-induced enzyme release may be consequential to muscle cell membrane permeability changes from decreased intracellular high-energy phosphates.