The present study examined the effects of dietary manipulations on six trained runners. The percent energy contributions from carbohydrate, fat, and protein were 61/24/14,50/38/12, and 73/15/12 for the normal (N), fat (F), and carbohydrate (C) diets, respectively. Expiratory gases and blood responses to a maximum (JOURNAL/mespex/04.02/00005768-199401000-00019/ENTITY_OV0312/v/2017-07-20T222405Z/r/image-pngO2max) and a prolonged treadmill run were determined following 7 d on each diet. Free fatty acids (FFA), triglycerides, glycerol, glucose, and lactate were measured. Dietary assessment of subjects' N diet indicated that they were consuming approximately 700 kcal·d−1 less than estimated daily expenditures. Running time to exhaustion was greatest after the F diet (91.2 ± 9.5 min, P < 0.05) as compared with the C (75.8 ± 7.6 min, P < 0.05) and N (69.3 ± 7.2 min, P < 0.05) diets. JOURNAL/mespex/04.02/00005768-199401000-00019/ENTITY_OV0312/v/2017-07-20T222405Z/r/image-pngO2max was also higher on the F diet (66.4 ± 2.7ml·kg−1·min−1, P < 0.05) as compared with the C (59.6 ± 2.8 ml·kg−1·min−1, P < 0.05) and N (63.7 ± 2.6 ml·kg−1·min−1, P < 0.05) diets. Plasma FFA levels were higher P < 0.05) and glycerol levels were lower (P < 0.05) during the F diet than during the C and N diets. Other biochemical measures did not differ significantly among diets. These data suggest that increased availability of FFA, consequent to the F diet, may provide for enhanced oxidative potential as evidenced by an increase in JOURNAL/mespex/04.02/00005768-199401000-00019/ENTITY_OV0312/v/2017-07-20T222405Z/r/image-pngO2max and running time. This implies that restriction of dietary fat may be detrimental to endurance performance.