A study to evaluate the long term effects of feeding a fat supplemented diet during medium intensity, aerobic training was conducted over 7 months using 12, 2-year-old Thoroughbreds. The control group (n=6) was fed grass hay and a fortified sweet feed (CON) and the other group (FAT)(n=6) received hay, sweet feed, a supplement pellet and 400 ml of soybean oil that supplied about 12% of the DE intake of the FAT group. After 2, 4 and 7 months, the horses performed 2 standardised exercise tests (SETs) on a high speed treadmill. The first test (STEP) consisted of sequential steps of 800 m at speeds of about 4, 8, 9, 10 and 11 m/s. V200 and VLA4 increased with training, but were not different between treatment groups. During the last STEP, insulin was higher post feeding in the CON horses. Blood glucose was lower at the end of the STEP in CON horses. The second test (SET30) consisted of 30 min of trotting at about 4 m/s. Cortisol was elevated during exercise in the CON horses during the last SET30. T4 was unaffected by diet or exercise. Consuming a fat supplemented diet altered the insulin response after feeding and this may have prevented a fall in blood glucose during strenuous exercise. Feeding a fat supplemented diet to 2-year-old. Thoroughbreds during training did not change red or white blood cell numbers or liver function as measured by serum levels of AST, γGT and SDH. Fat supplemented diets can be fed safely for extended periods of time to horses in training.