Abstract: Reasons for performing study: Intense exercise in horses induces an increase of creatine kinase (CK) and stimulation of neutrophils which release the strong oxidant enzyme, myeloperoxidase (MPO) into the blood. It is not known whether active MPO is found in equine muscles and whether oxidant activity of neutrophils could affect muscular tissues and mitochondrial activity. Objectives: Specific immuno-extraction followed by enzymatic detection (SIEFED) methods will be employed for the first time to assess both the normal range of MPO and mitochondrial complex I (MCI) activities in equine muscular microbiopsies and to study the variation of these activities induced by endurance races. Materials and methods: Forty-six microbiopsies were taken from 8 endurance Arabian horses (age: 102 years) in the triceps brachii (n=23) or in the gluteus medius muscle (n=23). Myeloperoxidase and MCI activities were measured in muscle extracts by enzyme immunocapture assays or SIEFED methods. Further, 7 endurance horses were sampled in the triceps brachii muscle before (T0) and after (T1) a 120 km endurance race (mean speed: 15.41.4 km/h). Results: The 46 microbiopsies from 8 horses revealed mean values for active MPO concentration and MCI activity of 2114 ng/mg proteins and 0.01720.0066 mOD/min/g proteins, respectively. No significant difference was observed between the 2 muscles. In 3 out of the 7 horses sampled after exercise, the 120 km endurance race induced a severe increase of muscle MPO activity (+11845% vs. T0), a large decrease of MCI activity (-6318% vs. T0) associated with a high mean plasma CK level (4642658 iu/l). In the 4 remaining horses, the 120 km endurance race did not modify the MPO and MCI activities and moderately increased the plasma CK level. Conclusions: Preliminary observations showed a possible link between MPO activity and mitochondrial functions.