In 10 dogs the authors studied the effect of increasing doses of alfathesin on ventilation (VE, frequency, VT, blood gases), on the ventilatory pattern (TI, TE, TI/Ttot), on the neurological initiation of ventilation (VT/TI, occlusion pressure at 0.5 seconds), and on the Hering-Beurer reflex (duration of apnoea after occlusion of the airway at the end of inspiration). The results were compared with normal values taken from the literature. The correlation between the dose of alfathesin and the measured or calculated parameters was examined. Ventilation was stimulated by low doses of alfathesin, a stimulation, characterized by tachypnoea without change in tidal volume. Deepening of anaesthesia was accompanied by progressively increasing depression of respiration (diminution of VE, of frequency, of VT/TI and increase of PaCO2 and of the duration of apnoea). The mechanisms of the initial stimulation of ventilation and of respiratory depression are discussed. The authors conclude that the action of alfathesin on the central nervous system is biphasic, with stimulation during light anaesthesia followed by depression with associated depression of ventilation, despite increasing hypoxia and hypercapnia.