The pulmonary arterial hypertension syndrome (PAH) in broilers due to hypobaric hypoxia generates big economic losses. We studied the effect of exposure to hypobaric hypoxia (2,638 m above sea level) in PAH development in Cobb 500 broilers maintained under controlled temperature conditions. These broilers had lower corporal weight than those raised under relative normoxic conditions (310 m above sea level). At 42 days of age the accumulative mortality was 35.2% in males and 53.8% in females; of these values, 90.6% in males and 96.4% in females were due to PAH. At 33 days of age,