Inhalation of nitric oxide (NO) and prostacyclin (PGI2) may induce selective pulmonary vasodilation and-by improving ventilation-perfusion ratio in ventilated areas of the lung-increase Pao2 in patients with acute lung injury. To assess the therapeutic efficacy of both compounds, dose-response curves were established in patients with adult respiratory distress syndrome (ARDS). Patients received both PGI2 (doses of 1, 10, and 25 ng/kg/min) and NO (concentrations of 1, 4, and 8 ppm). Cardiorespiratory parameters were assessed at control, at each drug concentration, and after withdrawal of NO and PGI2. PGI2 resulted in a significant, dose-dependent and selective reduction of pulmonary artery pressure (PAP) from 35.1 +/- 6.3 mm Hg at control to 33.1 +/- 4.8 (1 ng/kg/min), 31.3 +/- 4.8 mm Hg (10 ng/kg/min) and 29.6 +/- 4.5 mm Hg (25 ng/kg/min), respectively. Inhaled NO reduced PAP from 34.5 +/- 5.6 to 32.1 +/- 5.9 mm Hg at 4 ppm, and to 31.8 +/- 6.1 mm Hg at 8 ppm, respectively, with no effect at 1 ppm. Pao2/Flo2 ratio increased from 105 +/- 37 to 125 +/- 56 mm Hg (range of increase: 0 to 57 mm Hg) at PGI2 10 ng/kg/min and to 131 +/- 63 mm Hg (range: -5 to 89 mm Hg) at 25 ng/kg/min with no effect at 1 ng/kg/min. NO improved Pao2 (e.g., from 116 +/- 47 to 167 +/- 86 mm Hg at 8 ppm) and reduced intrapulmonary shunt at all doses tested. We conclude that both inhaled PGI2 and NO may induce selective pulmonary vasodilation and increase Pao2 in severe ARDS.