We tested the hypothesis that running down an incline, during which muscles primarily perform eccentric contractions, causes greater delayed-onset muscle soreness and greater increases in plasma enzyme activities than does running on the level, during which muscles perform similar amounts of concentric and eccentric contractions. Subjective sensations of muscular soreness and plasma activities of CPK and LDH were assessed in seven subjects at 0, 24, 48, and 72 h after 45 min of running (one time on the level and a second time down a 10% incline). Following downhill running (57% of VO2max), significant delayed-onset soreness was experienced in gluteal, quadricep, anterior leg, and posterior leg muscles, and plasma CPK (but not LDH) activity was significantly increased (351% at 24 h). In contrast, following level running (78% of VO2max), no statistically significant soreness occurred in any muscle group, and plasma CPK and LDH activities were not elevated. Thus, our results generally support the hypothesis. Secondarily, we investigated whether delayed-onset soreness with downhill running is accompanied by increases in peripheral white blood cell counts suggestive of inflammation. No such association was observed. We suggest that both delayed-onset muscular soreness and plasma enzyme activities are affected by structural changes in muscle tissue resulting from eccentric contractions.