Furosemide is a potent diuretic used in the horse for the prophylaxis of exercise-induced pulmonary hemorrhage(EIPH), and in a variety of clinical condition. The purpose of this study was to set up the effect of furosemide in Thoroughbred horses, and to examine the time of maximal effect of furosemide to prevent the EIPH. The animals of the experiment were 4 Thoroughbred racing horses(female, average 6 yrs, BW $507{pm}80.2kg$) in Jeju stud farm and the experiment was cross-over design between furosemide(1 mg/kg IV) and normal saline(0.1 ml/kg, IV) administration. We investigated the changes of clinical signs(body weight, skin turgor, capillary refilling time, jugular distensibility), CBC(PCV RBC), serum chemistry(TP, AST, GGT, glucose, LDH, BUN and creatinine) and electrolytes ($Na^+,;K^+,;Cl^-,;Ca^{2+}$) on 0, 30 mins, 1, 2, 4, 8 and 24 hrs in furosemide and saline administration roup, respectively. The weight decreased significantly(P<0.05) in 4 hrs after furosemide treatment, but skin turgor, CRT, jugular distensibility remained unchanged throughout the experimental period. RBC and PCV decreased after furosemide administration but soon recovered. TP increased to the highest level in 30 mins after furosemide administration and recovered in 8 hrs. Furosemide administration resulted in increases in glucose, AST, GGT, LDH, BUN, creatinine(P<0.05) and decreases in $Ca^{2+},;Na^+,;K^+,;Cl^-$ (P<0.05). All of values were within normal range throughout the experimental period. There was the alteration of blood and serum chemistry after furosemide administration, all of values were within normal range. And the best time of furosemide administration will be 30 minutes before the racing in order to preventing EIPH.