Intermittent positive pressure ventilation (IPPV) is useful method for compensate of respiratory function in anesthetized horses. However, IPPV may decrease cardiac output. Alterations in cardiac output of three groups (N = 5) healthy, halothane-anesthetized mares were determined and compared during a 120 min period of anesthesia. The groups were as follows: spontaneous ventilation (SV), controlled ventilation using an end-inspiratory pressure of 20 cmH2O (CV20) and a third group using 25 cmH2O (CV25) inspiratory pressure. In the CV groups, respiratory function was adequately maintained. Although, cardiac output tended to decrease over time in each group. After 105 min of anesthesia in the CV groups, the cardiac outputs decreased below 50 percent of pre-anesthetic values. In the CV25 group, cardiac outputs were significant difference (p < 0.05) from the SV group after 45, 90 and 120 min of anesthesia. There was no significant difference in cardiac output between SV and CV20 group over time. These values suggest that when long durations of anesthesia was used with the IPPV, decrease of cardiac output should be improved. In clinical use of halothane anesthesia, an end-respiratory pressure of 20 cmH2O seems to be appropriate because the mild decrease in cardiac output was observed even though a little high PaCO2.