OBJECTIVE: To evaluate the effects of hyperbaric oxygen solution on hypoxic pulmonary hypertension. METHODS: Eleven calves, 2-month-old, 71+/-6 kg, underwent general anaesthesia, mechanical ventilation and median sternotomy. Catheters for continuous pressure and blood gas measurements were inserted in carotid and femoral arteries, left atrium, right atrium and pulmonary artery (PA), and a flow-probe placed around the PA. After baseline measurements 30 min hypoxic ventilation reduced the mean arterial PO2 from 285+/-115 to 46+/-11 mmHg (P < 0.0001). At this point, without changes in hypoxic ventilation (mean arterial PO2 maintained at 50+/-5 mmHg), 3 ml/min of hyperbaric aqueous oxygen (AO, oxygen diluted in saline solution) was infused directly into the PA for 30 min, with continuous reading of the monitored parameters. RESULTS: Hypoxic ventilation raised significantly (P < 0.005) the values of systolic (36+/-7 vs 22+/-6 mmHg), diastolic (16+/-3 vs 9+/-4 mmHg) and mean (24+/-4 vs 14+/-4 mmHg) PA pressure, PA/systemic pressure ratio for systolic (0.47+/-0.09 vs 0.24+/-0.06) and mean (0.49+/-0.13 vs 0.23+/-0.08) pressures and Pulmonary Vascular Resistance (PVR) (6.89+/-0.87 vs 2.67+/-0.38 U), while the Pulmonary Blood Flow (PBF) decreased (2.7+/-0.4 vs 3.7+/-0.4 l/min). AO infusion reduced significantly (P < 0.005) the values obtained with hypoxic ventilation with systolic (26+/-6 vs 36+/-7 mmHg), diastolic (11+/-4 vs 16+/-3 mmHg) and mean (16+/-4 vs 24+/-4 mmHg) PA pressure, PA/systemic pressure ratio for systolic (0.27+/-0.07 vs 0.47+/-0.09) and mean (0.27+/-0.08 vs 0.49+/-0.13) pressures and PVR (3.42+/-0.31 vs 6.89+/-0.87 U), while the PBF increased (3.6+/-0.4 vs 2.7+/-0.4 l/min). CONCLUSIONS: Acute infusion of hyperbaric AO solution into the PA completely reverses the negative effects of acute hypoxia on pulmonary circulation.