We investigate the effects of training and detraining on the satellite cell activation in thoroughbred horse muscles after an exhaustive exercise. Six horses were subjected to conventional training for 18 weeks and detraining for 6 weeks. Before training (Pre), after 10-week and 18-week training (10Tr and 18Tr), and after 6-week detraining (DTr), an incremental exercise test (IET) was performed on inclined treadmill to measure VO₂max and the velocity at a plasma lactate of 4 mmol/l (VLA4). Biopsy samples from gluteus medius muscle was obtained before and at 1 minute (1min), 3 hour (3hr), 6 hour (6hr) and 1 day after each IET. Number of muscle satellite cell were counted in type identified muscle fibers by immuno-histochemical stain images. The levels of mRNA expressions were determined using real time RT-PCR system. The number of satellite cells in 10Tr was significantly higher in type IIa fibers (0.31±0.10) than Pre (0.15±0.06). As compared to each value before IET, IL-6 mRNA expression (fold change) increased remarkably at 6hr after IET in 10Tr (x 2290.2) and 18Tr (x 2304.2), but not in both Pre (x 260.0) and DTr (x 853.3). IGF-I and Myogenin mRNA expressions were significantly increased at 1 day after IET in 18Tr (x 6.6 and x 3.3), but not in both Pre and DTr. These results suggested that the increased reactivity of satellite cells by training for 18 weeks is almost disappeared after detraining for 6 week, as well as VO₂max and VLA4.