High-grain diets for the exercising horse were compared with diets which provided 15% of the total caloric intake from either vegetable oil or a highly fermentable fibre source (beet pulp). Six Thoroughbreds age 3 years were fed one of 3 diets or 5 weeks in a replicated 3 x 3 Latin square. The CONTROL diet was 3.65 kg of sweet feed (SF), 0.9 kg of a protein/vitamin/mineral pellet and 5.45 kg of hay cubes. The FAT diet replaced 1.15 kg of SF with 0.45 kg of soybean oil and the FIBRE diet replaced 1.15 kg of SF with 1.36 kg of beet pulp. Horses were exercised 3 times per week on a high-speed treadmill. During the last week of each period, the horses performed a standardised exercise test (SET). A series of blood samples was drawn immediately before feeding and every 0.5 h for 3 h after feeding, throughout the exercise bout and 30 min post exercise. Plasma was analysed for lactate, glucose, cortisol, insulin, packed cell volume, total protein and triglycerides. Water intake was measured at regular intervals during SET day. Blood glucose was lower (P < 0.05) in the FAT-fed horses during the 3 h post feeding as compared to either CONTROL or FIBRE-fed horses. Insulin was lower (P < 0.05) in the FAT-fed both post feeding and throughout exercise. Cortisol was lower (P < 0.05) in the FAT than the CONTROL-fed during exercise. Following exercise, the FAT-fed drank more water (P < 0.01) than either CONTROL or FIBRE-fed. Substituting 15% of DE as vegetable oil had a greater effect on metabolic response to exercise than a 15% substitution of beet pulp.