SUMMARY. To determine whether or not exposure to chronic hypoxia and subsequent development of pulmonary hypertension syndrome (PHS) induce alterations in endothelial nitric oxide (NO) production in broiler's pulmonary vascular bed of broilers, we studied the expression of nitric oxide synthase enzyme in pulmonary endothelial cells by a nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemical staining reaction. For this purpose, 60 broilers of three different ages (17, 30, and 42 days) were used. The animals were distributed in two groups: a) 30 healthy (nonhypertensive) broilers and b) 30 chicks with PHS. All broilers in group b had fewer NADPH-diaphorase-positive endothelial cells in arterioles than did the nonhypertensive broilers. These differences were highly significant (P < 0.01). These results demonstrate for, the first time in broilers, that hypoxia-induced pulmonary hypertension is associated with a decrease of endothelial-derived NO expression in pulmonary vessels.