Structural and functional changes of the pulmonary circulation, particularly during the pathogenesis of pulmonary arterial hypertension (PAH), remain to be fully elucidated. In this study, we utilized monochromatic synchrotron radiation (SR) microangiography to assess changes in pulmonary arteriole blood flow in the intact-chest rat after 4 wk of chronic hypoxia. Sprague-Dawley rats were exposed to normoxia (N-rats) or chronic hypoxia (10% O(2); CH-rats) for 28 days. Rats were anesthetized, and microangiography was performed on the left lung to assess 1) the branching distribution of pulmonary arteriole blood flow (internal diameter >80 microm) and 2) dynamic changes in vessel lumen diameter during acute hypoxic (8% O(2) for 4 min) pulmonary vasoconstriction (HPV) before and after beta-adrenoceptor blockade (2 mg/kg i.v. propranolol). Using SR angiography, we observed that the number of opaque third- and fourth-generation vessels (100-300 microm) for CH-rats was significantly fewer than the number for N-rats. The magnitude of HPV was not different between CH-rats and N-rats. Beta-adrenoceptor blockade accentuated the HPV in 200- to 300-microm vessels for CH-rats, but even more so in N-rats. However, in CH-rats, beta-adrenoceptor blockade also accentuated the HPV in 100- to 200-microm vessels. In summary, we utilized SR to assess gross blood flow changes and functional changes (i.e., HPV) of the pulmonary circulation in PAH. These results highlight the benefits of SR for assessing pulmonary circulatory pathology. Of particular importance, future use of SR will provide an effective method for assessing potential therapeutic treatments for PAH.