The purpose of this investigation was to determine the effects of moderate and heavy exercise on free fatty acid (FFA) mobilization and utilization in human subjects. Thirty-five male subjects performed two exercise tests on a motor driven treadmill. Fifteen minutes of moderate work (40 to 50% maxVO2) was followed by an additional 15 minutes of moderate work in one test, and by 15 minutes of heavy work (80 to 90% maxVO2) in the second test.Plasma glycerol concentration increased during both heavy and moderate exercise. Plasma FFA concentration decreased during the first 15 minutes of moderate exercise; when moderate exercise was continued for 15 more minutes, plasma FFA concentration rose toward the former resting level. When heavy exercise was performed, following 15 minutes of moderate exercise, the initial decline in plasma FFA concentration continued. During heavy exercise the percentage in plasma FFA of C18:2decreased, while that of C18:2increased. The opposite was true during moderate exercise. It is concluded that FFA mobilization and utilization were enhanced during both moderate and heavy exercise. FFA utilization during heavy exercise exceeded that during moderate exercise. Blood lactate concentration increased only when heavy exercise was performed and did not appear to inhibit FFA mobilization.