The effects of almitrine on pulmonary gas exchange and haemodynamics were compared to those of positive end-expiratory pressure (PEEP) in 10 patients with a severe adult respiratory distress syndrome (ARDS) who required continuous mechanical ventilation. Haemodynamic and gas exchange measurements were made before and after 30 min of PEEP at a level of 10 cmH2O, then 30 min later, before and at the end of the intravenous infusion of almitrine at a dose of 0.25 mg.kg-1 in 30 min. There was no significant difference between baseline gas exchange and haemodynamic parameters. PEEP and almitrine increased Pao (p = 0.001) from 10.9 to 12.6 kPa and from 10.6 to 12.6 kPa (1 kPa = 7.5 mmHg), respectively, and ratio of venous admixture to total blood flow (Qs/Qt) decreased (p less than 0.001) from 34 to 29% and from 33 to 29%, respectively, the effects of PEEP and almitrine being not significantly different. Neither PEEP nor almitrine caused a significant change in arterial carbon dioxide tension (PaCO2). The haemodynamic parameters did not change significantly with almitrine, whereas mean systemic arterial pressure decreased from 85.4 to 81.1 mmHg (p less than 0.05) with PEEP. These results are consistent with the hypothesis that both treatments improve ventilation/perfusion (VA/Q) distributions, by an increase in functional residual capacity in the case of PEEP and a redistribution of pulmonary perfusion in the case of almitrine.