Cardiopulmonary effects of spontaneous, assisted, and controlled ventilatory modes were determined with 6 young, healthy geldings anesthetized with halothane at a constant dose (1.3 minimum alveolar concentration). All horses were in lateral recumbency, and all modes of ventilation were studied at least once during each anesthetic exposure. Cardiac output did not differ between spontaneous and assisted ventilation modes, but both modes were associated with significantly (P less than 0.05) higher cardiac output than that with controlled ventilation. The PaCO2 differed significantly (P less than 0.01) between all modes of ventilation. Although controlled ventilation maintained a normal PaCO2, assisted ventilation reduced PaCO2 as compared with spontaneous ventilation with less cardiovascular depression than that with controlled ventilation. Mixed venous O2 tensions were higher with spontaneous and assisted ventilation modes than with controlled ventilation. Except for shorter inspiratory time and smaller inspiratory/expiratory ratio associated with spontaneous ventilation, there were no ventilatory mode-related effects on ventilatory variables.