The effect of warm-up exercise on energy metabolism and muscle glycogenolysis during sprint exercise (Spr) was examined in six fit Standardbred horses exercised at 115% of maximal O(2) consumption (VO(2 max)) until fatigued, 5 min after each of three protocols: 1) no warm-up (NWU); 2) 10 min at 50% of VO(2 max) [low-intensity warm-up (LWU)]; and 3) 7 min at 50% VO(2 max) followed by 45-s intervals at 80, 90, and 100% VO(2 max) [high-intensity warm-up (HWU)]. Warm-up increased (P < 0.0001) muscle temperature (T(m)) at the onset of Spr in LWU (38.3 +/- 0.2 degrees C) and HWU (40.0 +/- 0. 3 degrees C) compared with NWU (36.6 +/- 0.2 degrees C), and the rate of rise in T(m) during Spr was greater in NWU than in LWU and HWU (P < 0.01). Peak VO(2) was higher and O(2) deficit lower (P < 0. 05) when Spr was preceded by warm-up. Rates of muscle glycogenolysis were lower (P < 0.05) in LWU, and rates of blood and muscle lactate accumulation and anaerobic ATP provision during Spr were lower in LWU and HWU compared with NWU. Mean runtime (s) in LWU (173 +/- 10 s) was greater than HWU (142 +/- 11 s) and NWU (124 +/- 4 s) (P < 0. 01). Warm-up was associated with augmentation of aerobic energy contribution to total energy expenditure, decreased glycogenolysis, and longer run time to fatigue during subsequent sprint exercise, with no additional benefit from HWU vs. LWU.