This study evaluated 18 equines, disqualified only by exhaustion in endurance championship, by having completed 30, 60 or 90 km distance. Immediately after disqualification by exhaustion, the animals received thorough clinical examination. Hematocrit, total plasma protein, anion gap, excess/deficit of base, osmolality, and pH were measured, as well as analysis of the concentration of ions hydrogen, bicarbonate, sodium, potassium, calcium and chloride. Moderate hypovolemia due to dehydration resulting from the exercise was observed. The exhaustion caused by physical exertion altered the thermoregulatory mechanism, lowering the osmolality which was accompanied by sodium depletion. The distance run was not the determining factor for the occurrence of exhaustion; this condition was associated with the intensity of training.

Key words: Electrolytes, Endurance, Equus caballus, Exhaustion syndrome