The effect of oxygen breathing on the stiffness of the large pulmonary artery has not been elucidated. We analyzed the proximal pulmonary arterial impedance with a multisensor catheter in ten patients with pulmonary arterial hypertension (PAH), eight patients with pulmonary venous hypertension, and six control subjects. The stiffness of the vessel was quantified by the characteristic impedance (Zo) and compared with the plasma norepinephrine level. Ten minutes of high-oxygen breathing decreased the Zo (from 78 +/- 18 to 57 +/- 14 dynes.sec.cm-5, p less than 0.01) and pulmonary arterial resistance in all the cases with PAH. In this group, norepinephrine also decreased (from 381 +/- 89 to 319 +/- 77 pg/ml, p less than 0.01) following the correction of hypoxemia. Yet, those parameters did not change in the other two groups. These results indicate that in patients with PAH, oxygen breathing can reduce the stiffness of the main pulmonary artery because of the sympatholytic effect.