OBJECTIVE To compare the acute effects of shortly pre- vs. postprandial exercise on postprandial lipid metabolism in healthy but sedentary young Japanese women. METHODS Healthy young Japanese women with a sedentary lifestyle, normal weight (18.5 ≤ BMI < 25), normal ovarian cycle, and apolipoprotein E3/3 were selected as participants. A single bout of walking exercise was performed 20 min after (Exp. 1) or 50 min before (Exp. 2) the intake of oral fat tolerance test (OFTT) cream (1 g/kg body weight) at about 50% Vo(2)max for 30 min on a motorized treadmill. A control trial without exercise was also performed in each experiment. Each subject performed 2 trials in a randomized, cross-over design. Venous blood samples were drawn before the preprandial exercise (-1 h, Exp. 2 only) [corrected] and before (0 h) and 1, 2, 4 and 6 h after the fat intake for the determination of triglyceride (TG), apolipoprotein B-48 (apoB48), remnant-like particle-TG (RLP-TG), lactate, free fatty acid (FFA), insulin, and glucose. RESULTS In both experiments, postprandial serum TG concentrations in the exercise group were lower, but not significantly, than those in the control. In Exp. 1, incremental areas under the curve (IAUC) for TG and RLP-TG were slightly, but not significantly, smaller in the postprandial exercise group than the control. The values of apoB48, a marker of the chylomicron particle number, at 2, 4, and 6 h after the fat intake and IAUC for apoB48 were significantly lower in the postprandial exercise group than the control. In Exp. 2, IAUC for TG, RLP-TG, and apoB48 were not significantly different between the two groups. CONCLUSIONS The present findings suggest that postprandial, but not preprandial, exercise may reduce the number of chylomicrons and chylomicron remnants and improve exogenous lipoprotein metabolism. Postprandial exercise is more effective for improving postprandial lipoprotein metabolism than preprandial exercise.