Acute reductions in triglycerides and low density lipoprotein (LDL) cholesterol concentrations have been demonstrated in endurance athletes after prolonged exercise. To determine if similar changes occur in untrained subjects and to determine the duration of exercise necessary for such changes, we measured serum lipids and lipoproteins in 10 sedentary men after 1 hour of exercise at their anaerobic threshold. Findings in sedentary men were compared with those of 9 competitive cyclists after 1 and 2 hr of exercise. LDL cholesterol increased in the cyclists immediately after 1 and 2 hours of exercise. Total cholesterol and high density lipoprotein (HDL) cholesterol also increased in the cyclists immediately after the 2 hr session. These increases were transient and not significant when corrected for changes in plasma volume. Serum triglycerides were unchanged for 4 hr after exercise. By 24 hr, however, triglycerides had decreased in both the trained (17%) and untrained men (22%) after the 1 hr session and in the trained men (33% p less than 0.01) after the 2 hr session. These results demonstrate a delayed decrease in triglyceride concentration that is related to the duration of exercise and probably has no distinct threshold. The lower level of triglycerides in endurance athletes and in sedentary subjects after exercise training is due at least in part to an acute exercise effect.