A frequently used end point of clinical outcomes in patients with pulmonary arterial hypertension (PAH) is the 6-minute walk distance. Furthermore, some data suggest that mild to moderate exercise as an intervention in stable PAH is beneficial. Some of these questions have been recapitulated in the monocrotaline and hypoxia animal models of pulmonary hypertension. However, mild exercise and walk distance as end points have not been rigorously examined in the severe progressive Sugen 5416/hypoxia/normoxia (Su/Hx/Nx) animal model of PAH at each stage of worsening disease. Our hypothesis was that animals that were preselected as runners would have increased walk times and improved right ventricle/left ventricle plus septum (RV/LV+S) ratios, echocardiography, and histology compared with nonexercised Su/Hx/Nx animals. We examined four groups of rats: Su/Hx/Nx sedentary, Su/Hx/Nx exercised, control sedentary, and control exercised. Echocardiography was performed at 5, 8, and 13 weeks to assess right ventricular inner diameter in diastole and left ventricular eccentricity index. We found no difference between exercised and sedentary Su/Hx/Nx rats, and both were worsened compared with controls. Rats were euthanized at 13 weeks, and we found that neither RV/LV+S nor the occurrence of occlusive lesions were influenced by exercise. Most interesting, however, was that despite progressive PAH development, exercised Su/Hx/Nx rats showed no decrease in time or distance for treadmill exercise. In all, our data suggest that, despite severe PAH development, Su/Hx/Nx rats retain the same treadmill exercise capacity as control animals.