 Specific drugs affecting pulmonary vascular resistance have been studied. Intravenous prostacyclin has major disadvantages: high cost, tachyphylaxis, risk of infection and rebound hypertension upon discontinuation. Inhalative pulmonary vasodilators, in particular iloprost, may be effective in primary pulmonary hypertension Olschewski, Ann Int Med 1996; Hoeper, Pneumologie 2001, but administration is time-consuming, and due to its mode of application its effects are intermittent, lasting only about 75 minutes Hoeper, JACC 2000. Considering this, oral treatments appear preferable, because of easy administration and, hence, better patient compliance. 