Endurance horses are usually eliminated from the race due to lameness, metabolic ailments or technical reasons. The purpose of the study was to determine the validity and reliability of the metabolic disorder index (MDI) in predicting metabolic eliminations in endurance horses during an endurance race. Fifty-four endurance horses competing on two local endurance tracks were involved in the study. Blood samples were collected a day prior to the event to determine packed cell volume (PCV), chloride (Cl-), interleukin-6 (IL-6), creatine kinase (CK) and glutathione reductase (GR) concentrations from all participating horses. These parameters were used for the determination of metabolic disorder index in endurance horses at rest (one day before the competition). All data were statistically analysed. In 40, 80 and 120 km race distances, the successfully completed horses had a significant lower serum concentration of CK and a significant higher serum concentration of Cl- than the eliminated horses (P