Following the VO2 max test, subjects will return to ground level for a 3-hour rest period. At 90 minutes subjects will be administered riociguat 0.5 mg or 1.0 mg orally. Once study subjects are at therapeutic levels of riociguat (30 to 90 minutes after oral administration), they will repeat the VO2 max test at 15,000 feet. The dosing of riociguat will start at the lowest recommended individual dose (0.5 mg) for the first three subjects. If there are no side effects and no clinically important difference in either PAP (5 mmHg decrease in mean PAP) or PaO2 (5 mmHg increase) during exercise, then for the remaining subjects the dose will be increased to 1.0 mg.