Almitrine bismesilate (Vectarion) is characterised by its specific activity on carotid and aortic chemoreceptors. A simultaneous study of the ventilatory and blood gas changes in the anaesthetised dog has shown a dissociation between these two effects. At a dose of 10 to 30 micrograms/kg IV Almitrine bismesilate induces a significant rise in the PaO2 whereas no change in the ventilatory output is observed. A significant transitory rise in the pulmonary arterial pressure also appears at these doses. The juxta position of these results suggests that a feeble doses Almitrine bismesilate improves the blood gases of the arterial blood by a re-distribution of the pulmonary blood flow. Clinical pharmacological studies carried out in bronchitic patients have shown an improvement in the distribution of ventilation perfusion ratios VA/Q by Almitrine bismesilate. These experiments were carried out in animals in order to determine whether the drug had a preferential action on one or other side of the VA/Q ratio. Whereas in an animal with intact lungs, Almitrine bismesilate potentiates hypoxia induced vaso-constriction, in animals whose lungs are ventilated in a heterozygous fashion in such a way as to reproduce zones with a shunt effect, the potentiation of hypoxic vasoconstriction was not again found. At low doses in the hypoxic rat Almitrine bismesilate increases alveolar ventilation (VH) by a significant rise in tidal volume and inspiratory output (VT/TI) without increasing external ventilation. Overall the rise of PaO2 induced by Almitrine bismesilate at low doses may be accompanied by a similar effect on the pulmonary circulation noted in the anaesthetised dog. In the hypoxic rat, it is changes in the alveolar ventilation that are observed.(ABSTRACT TRUNCATED AT 250 WORDS)