Nine Standardbred horses of similar athletic fitness (six mares, three geldings), ranging from 4 to 11 years of age, were used to determine the effects of 0, 250, or 500 mg intravenously administered furosemide on plasma tCO2 changes over time. All horses were either cur- rently racing or in advanced stages of race training be- fore entering a qualifying race. Horses were randomly allotted to one of the three treatment levels of furosemide during 3 consecutive weeks. Jugular venous samples were obtained from horses at rest in box stalls before and hourly for 6 hours after administration of furosemide. Body weights of horses ranged from 356 to 456 kg, and the mean was 417 kg. Thus, the dose of furosemide received by each horse ranged from 0.55 to 0.70 mg/kg body weight for the 250-mg injections and from 1.1 to 1.4 mg/kg body weight for the 500-mg injec- tions. Furosemide caused metabolic alkalosis in the horses. Least square means (±SEM) were determined and horses had adjusted plasma tCO2 of 32.2, 33.9, and 34.7 ± 0.41 for the 0-, 5-, and 10-mL doses of furosemide, respectively.The type 3 tests of hypotheses found that there was a difference (P < .0001) across time, a difference (P = .0016) according to furosemide dose, and a difference (P < .0001) according to treat- ment ϫ hour.There was no difference (P > .05) accord- ing to week or treatment ϫ week. These data sugges