To study how diet composition affects exercise endurance and body composition, 48 male Sprague-Dawley rats were treadmill trained for 8 wk while consuming either a high-fat (F) diet or high-carbohydrate (C) diet. The diets were switched for one-half the number of rats in each group 3 days before the animals were killed, during which feeding time the rats did not exercise. One-half of rats receiving each of the four diet combinations were taken at rest (R) or exhaustion (E), resulting in eight groups: CCR, CFR, FFR, FCR, CCE CFE, FFE, and FCE. An analysis of variance revealed that resting glycogen in the FCR group was enhanced in muscle (19-33%) and liver (23%) compared with controls. Each F group's exercise time to exhaustion [CFE, 322.9 +/- 25.0; FFE, 356.8 +/- 37.8; FCE, 467.0 +/- 32.6 (SE) min] was different (P < 0.05) from control (CCE, 257.5 +/- 29.2 min). Postexercise glycogen was equivalent among all dietary groups, were muscle triglycerides. The FF and FC groups had higher 3-hydroxyacyl-CoA dehydrogenase activity in soleus muscle than either CC or CF animals. After training, body weights were similar between the two dietary groups; however, percent body fat was 17% greater after the F diet, even though F diet animals voluntarily consumed 12% less energy than did C diet animals. These data suggest that exercise endurance time is optimized in trained rats that receive a carbohydrate load after adaptation to a F diet. However, despite intense exercise training, the F diet promotes body fat deposition, and the health consequences of following such a regimen are still unknown.