Morphological and physiological responses to unilateral pulmonary artery occlusion (PAO) were evaluated in male broiler (B) and Leghorn (L) chickens. All birds were fed a diet containing 3,200 kcal of ME/kg of feed and 23% CP. Broilers (18-21 d old; 507 ± 40 g of BW) and L (61-64 d old, 861 ± 87 g of BW) had surgical PAO (n = 40 each strain) or were sham-operated (SHAM; n = 40 each strain). Hematocrit (%, Hc), relative lung weight (wet right + left lung weight/BW × 100), right ventricle to total ventricle weight ratio (RV/TV), and resistance pulmonary arterioles (RPA) thickness were measured in 6 chickens per group one day presurgery, and at 7 and 14 d postsurgery. Data were analyzed using one-way ANOVA. Relative lung weight was higher in L-chickens than in B-chickens at all sampling times. There were no differences in Hc between B and L presurgery, but at d 7 and 14, L-PAO chickens had the highest Hc (35 ± 1.4 and 40 ± 1.9, respectively); the B-SHAM had a lower Hc (28 ± 1.2, and 29 ± 1.0) than the L-SHAM (32 ± 1.2 and 34 ± 1.1) and the B-PAO (32 ± 1.2 and 34 ± 2.0) chickens, with no differences between L-SHAM and B-PAO. The RV/TV ratio was highest in the B-PAO at d 7 and d 14, with no differences among the other groups. The B-PAO chickens had the thickest RPA at 7 d and 14 d postsurgery than the rest of the groups, whereas B had thicker RPA than L at presurgery. Broilers had a lower ventilation capacity than L, and after PAO they developed right ventricular hypertrophy and small arteriole remodeling, whereas the L-PAO showed a higher degree of hypoxemia (high Hct), but without changes in RV/TV ratios or small arterial remodeling, suggesting that L-chickens had a better pulmonary arterial vasodilation even after chronic increases in blood flow through a single lung.