In brief: The authors tested the hypothesis that delayed-onset muscular soreness after running is related to the production of lactic acid during the exercise. Blood lactic acid concentration was measured before and during 45 minutes of treadmill running, one time on the level and once at a-10% incline. Blood lactic acid concentration and subjective sensations of muscular soreness were assessed at intervals for 72 hours after the runs. Lactic acid concentration was significantly increased during running on the level, but subjects experienced no significant postexercise muscular soreness. Lactic acid was never elevated in downhill runners, but subjects experienced significant delayed-onset soreness. Results indicated that lactic acid is not related to exercise-induced delayed-onset muscle soreness.