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OBJECTIVE To evaluate changes in electrolyte concentrations and hydration status that take place in endurance horses prior to the start of a competition and determine whether these changes would be associated with elimination. ANIMALS 19 horses entered in the 2016 Tevis Cup 100-Miles (160 km) One-Day Western States Trail Ride. PROCEDURES Heparinized blood samples were collected at 5 time points: prior to transport to the ride (TO), during check-in the day before the ride (TI), 1 to 2 hours before the start of the ride (TZ), at the 15-km mark (T3), and at the 55-km mark (T4). Packed cell volume and plasma sodium, potassium, chloride, urea nitrogen, glucose, bicarbonate, and total protein concentrations were determined and compared across time points and between finishers and nonfinishers. RESULTS Significant differences were detected among plasma sodium, potassium, and urea nitrogen concentrations measured priorto the start of the ride (ie, TO, TI, and T2). For all variables except chloride and bicarbonate concentrations, significant differences were detected between values obtained prior to the start of the ride and values obtained during the ride (ie, T3 and T4). Only bicarbonate concentration at the 15-km mark of the ride was significant: ly associated with finishing status. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that significant changes in plasma sodium, potassium, and urea nitrogen concentrations can occui-in endurance horses during transport to a competition and when horses are statled overnight before an event. Additionally, a lower bicarbonate concentration following a steep climb early during the ride was associated with subsequent elimination. [ABSTRACT FROM AUTHOR]