Changes in venous blood pH, pCO2, pO2, HCO-3 and cBase, and in serum concentrations of sodium, potassium, ionized calcium and chloride were studied in eight Arabian horses during endurance ride of 60km, with controlled speed of 12km/h-. The blood samples were collected prior to the beginning (M0) of the exercise, immediately after the ride (M1) and one hour after the ride (M2), with the animals resting. Immediately after the end of the exercise (M1) the animals presented higher values for pH which, added to the increase of cHCO3- and cBase, characterized the metabolic alkalosis, probably due to losses of chloride ions in the sweat. The metabolic changes observed in M1 were quickly corrected by respiratory changes, characterized by increased pCO2 or hypoventilation. Additionally, the horses presented dehydration, hypocalcemia and hypokalemia. The metabolic disturbances observed were not associated with clinical changes. KEY WORDS: Acid-base, electrolytes, endurance, equine