This study investigated the effects of increased fat consumption on aerobic and anaerobic performance. Eight Thoroughbred horses that had undergone a 9 week, 6 days/week conditioning programme were randomly divided into 2 groups, Group F (5 horses) and Group C (3 horses). For 2 weeks before the study all horses consumed a standard diet (2% fat) fed as 2 equal portions. Horses then undertook an incremental treadmill exercise test (+10% slope); 3 min at 4 m/s, 2 min at 6 m/s and 1 min at 8 m/s. Speed was then increased by 1 m/s every 60 s until fatigue. Maximal accumulated oxygen deficit (MAOD) was determined 72 h later. Horses in Group F were then fed a diet containing 12% of the digestible energy in the form of fat (390 ml corn oil), a diet that was isocaloric with the control diet. Horses in Group C continued to receive the standard diet. All horses continued training (3600 m/day, at speeds up to 10 m/s, 6 days/week). After 4 weeks, the exercise tests were repeated. Prior to fat supplementation, V̄O2max was mean ± s.e. 145 ± 5.3 and 148.0 ± 6.0 ml/kg/min, time to fatigue during the MAOD test was mean ± s.e. 84.2 ± 18.4 and 85.0 ± 14.8 s, mean ± s.e. peak plasma [lactate] was 24.8 ± 3.1 and 23.5 ± 1.7 5 mmol/l and MAOD was 56.5 ± 5.3 and 68.1 ± 4.7 mlO2eq/kg for Groups F and C respectively. The V̄O2max and run time to fatigue in the incremental exercise test did not change in either group following fat supplementation. Run time (mean ± s.e. 97.2 ± 17.8 s), MAOD (65.5 ± 6.2 mlO2eq/kg) and peak plasma [lactate] (29.5 ± 2.6 mmol/l) increased during the MAOD test in Group F but remained unchanged in Group C. Resting muscle [glycogen] depletion during the MAOD test was not altered by the dietary manipulation. Provision of an increased proportion of the diet in the form of fat increased high intensity exercise capacity and the MAOD during intense treadmill exercise.