To evaluate the potential efficacy of the combined administration of inhaled nitric oxide and oral beraprost sodium in producing pulmonary vasodilation, we studied 20 patients with pulmonary hypertension during cardiac catheterization. We also evaluated some of the mechanisms of vasodilation under these circumstances by investigating the levels of cyclic GMP and AMP in the pulmonary venous blood. A significant decrease in pulmonary vascular resistance was observed after the administration of nitric oxide (−37 ± 6%, p<0.01), and more intense decrease was observed after the combined administration of nitric oxide and beraprost sodium (−46 ± 6%, p<0.05). In addition, the ratio of pulmonary-to-systemic resistance also decreased to a greater extent with combined administration than with nitric oxide alone (−34 ± 6% vs −42 ± 7%, p<0.05). Conversely, systemic vascular resistance showed no change, neither after loading with nitric oxide nor after combined administration. Although equivalent increases in levels of cyclic GMP were observed after inhalation of nitric and the combined administration (mean 78% vs 65%), greater increases in levels of cyclic AMP were observed with the combined use (mean 15% vs 69%). Combined treatment with nitric oxide and beraprost sodium may prove markedly beneficial in patients with primary and postoperative pulmonary hypertension who do not respond adequately to inhalation of nitric oxide alone or conventional modes of treatment.