Repeated episodes of exaggerated postprandial lipemia may hasten the progression of atherosclerosis. The purpose of this study was to compare the lipemic response to a high-fat meal in trained and untrained women in the presence and absence of the acute effects of exercise. Nine endurance-trained and thirteen untrained women aged 40.4 +/- 3.3 and 43.8 +/- 4.3 y (mean +/- SD), with maximal oxygen uptake of 50.3 +/- 5.9 and 31.7 +/- 3.6 mL.kg-1.min-1, and a body mass index (kg/m2) of 22.2 +/- 0.9 and 22.9 +/- 2.3, respectively, underwent two trials, each over 2 d. Subjects did not exercise during the 2 d leading up to a trial. On day 1 they either walked for 90 min at 60% of maximal oxygen uptake (exercise), or refrained from exercise (control). On day 2 venous blood and expired air samples were obtained in the fasted state and for 6 h after consumption of a high-fat meal (1.70 g fat, 1.65 g carbohydrate, and 0.25 g protein/kg fat-free mass). Exercise decreased lipemia as determined by the mean (+/-SEM) area under the plasma triacylglycerol concentration versus time curve: trained, 6.96 +/- 0.48 compared with 4.87 +/- 0.33 mmol.h/L; untrained, 8.36 +/- 0.83 compared with 7.01 +/- 0.79 mmol.h/L (control and exercise trials, respectively, both P < 0.05). Lipemia differed significantly between groups in the presence of this acute effect of exercise but not in its absence. Exercise decreased insulinemia in trained women (543 +/- 25 compared with 433 +/- 24 pmol.h/L, P < 0.01) but had no effect in untrained women (592 +/- 34 compared with 585 +/- 47 pmol.h/L). Total oxidation of fat over the 6-h postprandial period was enhanced by exercise, and to a similar degree in each group of women.