The aim of the study was to assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of BAY 63-2521, a new drug in development for pulmonary hypertension. Fifty-eight healthy male volunteers received a single oral dose of BAY 63-2521 (0.25-5 mg) or placebo. No serious adverse events were reported; there were no life-threatening events. Heart rate over 1 minute, an indicator of the effect of a vasodilating agent on the cardiovascular system in healthy subjects, was increased dose dependently versus placebo at BAY 63-2521 doses of 1 to 5 mg (P < .01). Mean arterial and diastolic pressures were decreased versus placebo at doses of 1 mg (P < .05) and 5 mg (P < .01). Systolic pressure was not significantly affected. BAY 63-2521 was readily absorbed and exhibited dose-proportional pharmacokinetics. The pharmacodynamic and pharmacokinetic properties of BAY 63-2521 suggest that it can offer a unique mode of action in the treatment of pulmonary hypertension.