Description: To assess the accuracy of an ultrasound velocity dilution cardiac output (UDCO) method, compared with that of the lithium dilution cardiac output (LiDCO) method, for determination of cardiac output (CO) in juvenile horses with experimentally induced hypovolemia. Animals--12 anesthetized 2-to 6-month-old horses. Procedures--For each anesthetized horse, CO was determined by the LiDCO and UDCO methods prior to any intervention (baseline state), after withdrawal of approximately 40 percent of the horse's blood volume (low CO state), after maintenance of hypovolemia and infusion of norepinephrine until mean arterial blood pressure was equal to baseline value (high CO state), and after further infusion of norepinephrine and back-transfusion of withdrawn blood (posttransfusion state). For each of the 4 hemodynamic situations, CO and calculated cardiac index (CI) values were obtained by each method in duplicate (8 pairs of measurements/horse); mean values for each horse and overall mean values across ail horses were calculated. Agreement between CI determined by each method (96 paired values) was assessed by Bland-Altman analysis. Results--For the UDCO method-derived CI measurements among the 12 horses, mean с SD bias was -4 с 11.3 mL/kg/min (95 PROCENT limits of agreement, -26.1 to 18.2 mL/kg/min) and mean relative bias was -10.4 с 21.5 percent 95 percent limits of agreement 52.6 percent to 31.8 percent . Conclusions and Clinical Relevance--Results indicated that, compared with the LiDCO method, the UDCO method has acceptable clinical usefulness for determination of CO in foals