Almitrine bismesylate has been shown to be a respiratory stimulating substance which acts primarily via the peripheral chemoreceptors. The effect of the oral administration of Almitrine (200 mg/day for 9 days) was investigated in a controlled double-blind study with 18 patients suffering from chronic obstructive lung disease and hypoxemia (PaO2 less than 60 mm Hg). During the therapy with Almitrine an early and lasting improvement in the PaO2 and PaCO2 could be obtained. This improvement was significant compared with the placebo group. On the last day of treatment with Almitrine the PaO2 increased by 16.2 mm Hg (p less than 0.001) and the PaCO2 decreased by 7.3 mm Hg (p less than 0.01). Simultaneously, an increase in minute ventilation (5.24 liters on day 9) could be observed. This increase was not significant compared with the placebo group. There was a correlation of -0.61 between the changes in minute ventilation and in PaCO2. The lung functional parameters vital capacity (VC), forced expiratory volume in 1 s (FEV1) and peak expiratory flow (PEF) increased on several days in both groups; a change in Rtot was observed in the treatment group only. A reduction of dyspnea also was observed in both groups. In addition to the ventilatory stimulant action on the peripheral chemoreceptors, the changes in blood gases induced by Almitrine also should be discussed as an improvement of the ventilation/perfusion ratio. In this therapeutical trial no side effects during the therapy with Almitrine have been observed.