To compare the cardiopulmonary effects of the head-down position, with or without capnoperitoneum, in halothane-anesthetized horses.Prospective randomized study.Five ponies (four mares, one stallion; bodyweight 302 ± 38.4 kg [mean ± SD]) were used.The ponies were anesthetized with xylazine, guiafenesin, ketamine, and maintained with halothane/oxygen and lungs were ventilated to 40 ± 2 mm Hg (5.3 ± 0.3 kPa) end-tidal CO2 tension. After baseline cardiopulmonary measurements, ponies were kept in horizontal position for 30 minutes, then tilted head-down 30° to the horizontal position for 60 minutes, and then returned to a horizontal position for final measurements. Capnoperitoneum (intra-abdominal pressure: 12 mm Hg [1.6 kPa]) was introduced after baseline cardiopulmonary measurements, until 5 minutes before the final measurements (treatment INS). Ponies in the control treatment (CON) did not receive capnoperitoneum. Cardiopulmonary data were collected every 10 minutes following the baseline measurements until recovery.In the head-down position, in both treatments, significant decreases were observed in PaO2, and significant increases were observed in PaCO2, right atrial blood pressure, arterial to end-tidal CO2 gradient, calculated Vd/Vt and Q˙s/Q˙t ratios. During the head-down position, in CON there was decreased cardiac index, and in INS, there were decreases in arterial plasma pH and increases in mean systemic arterial and airway pressures. Treatment INS developed ventilation-perfusion mismatch earlier in the study, and had longer recovery times compared to CON.Cardiac index and systemic blood pressure appeared to be preserved in INS during the head-down position, but ventilation-perfusion mismatch appeared earlier with head-down position and capnoperitoneum.Healthy ponies tolerate capnoperitoneum at 12 mm Hg (1.6 kPa) intra-abdominal pressure when tilted head down 30° to the horizontal position.