Four Quarter Horses (1 filly age 2, 1 mare age 5 and 2 geldings ages 3 and 4; average weight 539 kg) were used in a 2 x 2 crossover design. The effects of furosemide (Lasix(Rx)) on arterial blood packed ceii voiume (PCV), hemogiobin (Hb), pH, pO2, pCO2, HCO-3 and base excess (BE) were measured. Plasma lactate, heart rate, and fatigue time were determined as indicators of perlormance while the horses performed a fatigue test on a high-speed treadmill. The left carotid artery was surgically elevated subcutaneously to facilitate collection of arterial blood samples. Horses were conditioned for 13 weeks with increasing intensity then randomly assigned furosemide (F) or physiological saline (C) as treatments. Treatments were administered 4 hours prior to the fatigue test in accordance with racing regulations. Arterial blood samples were collected prior to treatment dose, prior to exercise, at the 2nd, 4th, and 6th minute during the fatigue test, at fatigue, and at the 5th, 15th, 30th, and 45th minute post-exercise. Arterial blood samples were analyzed for blood gases, Hb, PCV, and plasma lactate. Heart rate and fatigue time were recorded. No difference between treatments (P > 0.05) was observed for blood gases except for pCO2 at rest, and HCO-3 and BE at the 2 minute collection period. No difference between treatments (P > 0.05) was observed for Hb, PCV, lactate and heart rate except at 15 minutes post-exercise for Hb and PCV, and 45 minutes postexercise for Hb. Fatigue times were 11 min 56 sec ± 5 min 30 sec for F horses and 11 min 35 sec--± 2 min 6 sec for C horses. No difference (P > 0.05) was observed in fatigue time. Based on our data, the trend indicated that all parameters measured returned to pre-exercise levels more rapidly for furosemide treated horses. However, furosemide did not enhance performance.