To compare the ability of bioreactance noninvasive cardiac output (BR-NICO) with thermodilution cardiac output (TDCO) for the measurement of cardiac output (CO) in healthy adult horses receiving 2 different IV volume replacement solutions. Prospective randomized crossover study from September to November 2021. University teaching hospital. Six university-owned adult horses. Horses were randomly assigned to receive 7.2% hypertonic saline solution (HSS) or 6% hydroxyethyl starch (130/0.4) solution (HETA). BR-NICO and TDCO were measured simultaneously at baseline, 10, 20, 30, 40, 50, 60, 90, and 120minutes before fluid administration and again at the same times after starting a bolus infusion of IV volume replacers. All solutions were administered within 10minutes. BR-NICO and TDCO were strongly correlated (Pearson r2=0.93; P<0.01). Regression analysis showed the relationship between TDCO and BR-NICO was BR-NICO=0.48×TDCO+0.6. For the corrected BR-NICO values, the Bland-Altman agreement mean bias and lower/upper limits of agreement were -0.62 and -5.3 to 3.9L/min, respectively. Lin's concordance (95% confidence interval) between methods was 0.894 (0.825-1.097). Compared with baseline, HSS increased the CO at 10 and 20minutes (TDCO and BR-NICO). Compared with baseline, HETA decreased the CO at 10 and 20minutes (TDCO and BR-NICO) and increased the CO at 90 (TDCO only) and 120minutes (TDCO and BR-NICO). BR-NICO strongly agreed with TDCO in the measurement of CO in healthy unsedated adult horses. HSS administration significantly increased CO in the first 30minutes, while HETA initially decreased CO at 10minutes but increased CO to above baseline values by 90minutes.