We examined the effectiveness of inhaled nitric oxide (NO) as a selective pulmonary vasodilator in acute pulmonary hypertension in an in vivo canine model with fixed cardiac output. In 5 dogs, total right heart bypass was instituted, and pulmonary hypertension was induced by infusion of the thromboxane analogue U-46619. During U-46619 infusion, NO was administered at 10 and 40 ppm for 5 minutes followed by breathing of the oxygen mixture without NO. Pump flow was held constant during the experiment. Infusion of the thromboxane analogue resulted in an increase in pulmonary vascular resistance and systemic vascular resistance from 147 +/- 83 to 740 +/- 126 dyne.s.cm-5 and from 1,720 +/- 113 to 2,407 +/- 232 dyne.s.cm-5, respectively. During inhalation of 10 ppm NO, pulmonary vascular resistance significantly decreased to 613 +/- 55 dyne.s.cm-5 (p < 0.05) and further decreased to 527 +/- 163 dyne.s.cm-5 with 40 ppm NO inhalation (p < 0.05). Systemic vascular resistance did not change during NO treatment (2,300 +/- 70 dyne.s.cm-5 during 40 ppm NO). There was no increase in intrapulmonary shunting or methemoglobin levels during NO inhalation. In this setting, with a constant cardiac output throughout the experiment, NO acted as a selective pulmonary vasodilator without altering systemic vascular resistance. However, induced pulmonary vasoconstriction was only partially reversed by NO inhalation.