Objective To investigate the effect of carbon monoxide (CO) inhalation on acute hypoxic pulmonary arterial hypertension and acute hypoxic lung injury. Methods Thirty Wistar rats weighing 180-250 g were anesthetized with intramuscular 3% pentobarbital 50 mg?kg-1 . Pulmonary catheter was inserted via right external jugular vein for mean pulmonary arterial pressure (MPAP) monitoring. Left common carotid artery was cannulated for BP monitoring. Tracheotomy was performed and the animals were mechanically ventilated ( RR = 80-90 bpm). PETCO2 was maintained at 35-45 mm Hg. The animals were randomly divided into 3 groups: (1) control group inhaled 21 % O2 for 30 min ( n = 10); (2) hypoxia group inhaled 10% O2 + 90% N2 for 30 min ( n = 10) and (3) CO group inhaled 100 ppm CO in 10% O2 + 90% N2 for 30 min ( n = 10) . Arterial blood 4 ml was obtained at the end of experiment for estimation of blood HbCO and plasma MDA concentration and RBC SOD activity. A piece of lung tissue was obtained immediately after the animal was sacrificed for electron microscopic examination. Results (1) In hypoxia group MPAP was markedly increased, plasma MDA level elevated and SOD activity reduced as compared with control group ( P