Almitrine bismesylate is a new, orally administered, respiratory stimulant that improves arterial blood gas tensions in patients with chronic bronchitis and emphysema, and it may have an effect on the pulmonary circulation and on right ventricular performance. We have, therefore, compared the effects of Almitrine with those of oxygen (given as 3 L/min by nasal prongs) on arterial blood gas tensions, mean pulmonary arterial pressure (Ppa), and right ventricular ejection fraction (RVEF) measured both at rest and during exercise in patients with chronic hypoxemia caused by chronic bronchitis and emphysema. Arterial oxygen tension improved significantly both at rest and during exercise after either 100 mg Almitrine by mouth or when breathing oxygen, both at rest and during exercise. Almitrine increased the mean Ppa at rest from 22 +/- 4 to 35 +/- 5 mmHg (p less than 0.001), and mean Ppa rose further during exercise from 38 +/- 5 mmHg before Almitrine to 49 +/- 7 mmHg (p less than 0.001) after Almitrine. In contrast, oxygen breathing did not change Ppa when at rest but reduced the amount of rise in Ppa during exercise. The change in Ppa after Almitrine correlated with the plasma Almitrine concentration (r = 0.69, p less than 0.05) and was associated with a fall in RVEF at rest from 0.38 +/- 0.03 to 0.32 +/- 0.02 (p less than 0.001). In 5 of the patients who received 50 mg of Almitrine by mouth twice daily for 3 months, the Ppa both at rest and during exercise remained significantly higher than in the control study before receiving Almitrine, but RVEF was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)