There is direct evidence in dogs and circumstantial evidence in man that almitrine bismesylate stimulates peripheral chemoreceptors. In anaesthetized dogs, almitrine bimesylate causes an increased activity in afferent fibres from the sinus nerve, in inspiratory neurons of the nucleus tractus solitarius, in expiratory neurons of the nucleus retroambigualis, and in the phrenic nerve. The mechanism whereby almitrine bismesylate stimulates chemoreceptors is poorly understood to date. As a result of chemoreceptor stimulation by almitrine bismesylate , ventilation is higher or unchanged even though PaCO2 has fallen and PaO2 has risen. However, in many patients with chronic obstructive lung disease given almitrine bismesylate , the improvement in arterial blood gases cannot be fully explained by the increased ventilation. This suggests an improved matching of ventilation and perfusion that could be due to diversion of blood flow from poorly towards better ventilated zones. However, all animal experiments have failed to demonstrate such a diversion. Redistribution of ventilation could also explain the improved V/Q matching but has not been unequivocally proven.