The relationship between fiber type composition and the increase in blood ammonia was examined following a maximal O2 consumption (VO2max) test. Muscle biopsies were taken from the middle portion of the vastus lateralis for determination of fiber type percentages. Two subject groups were selected on the basis of a high (HST) or low (LST) percentage of slow-twitch fibers and compared for blood ammonia and lactate levels after exercise at work loads of approximately 85 and 110% of VO2max. An inverse relationship was found between the percentage of slow-twitch fibers and the increase in blood ammonia. Blood ammonia increased after exercise at both 85 and 110% of VO2max. However, the increase was twofold greater for the LST group following the 110% work effort. The increases in blood ammonia and lactate were positively correlated for both groups following exercise. The results suggest that the proportion of slow-twitch fibers plays an important role in determining the magnitude of the increase in blood ammonia after intense exercise.