In order to estimate the long-term hemodynamic and histologic effects of bunazosin hydrochloride on primary pulmonary hypertension (PPH), for which effective internal therapy has not yet been proposed, we produced subacute pulmonary hypertension in 6 week old Sprague-Dawley rats by injecting one dose of monocrotaline. Bunazosin (10 mg/kg) was administered intraperitoneally every day and bodyweight and hemodynamic parameters were measured. Right ventricle (RV) and left ventricle with septum (LV +S) were weight separately. Medial thickness of the small pulmonary artery was calculated. In the bunazosin group, the increase of right ventricular systolic pressure (RVSP) as pulmonary arterial systolic pressure and the ratio of RVSP to pulmonary flow were limited, without causing systemic hypotension, compared to the control group. Weight ratio of RV/(LV +S) had a tendency to be limited in the bunazosin group. As for medial thickness, there was no significant difference between the two groups. It may be concluded that bunazosin hydrochloride should be considered for clinical trials in treatment of PPH in humans.