Many anaesthetic agents depress respiration and this can lead to the production of hypercapnia, hypoxia and acidosis. To maintain blood carbon dioxide and oxygen concentrations within normal levels, it is often necessary to assist ventilation. If the thoracic cavity is opened, the normal mechanisms of lung inflation are disrupted and it is usually necessary to ventilate the animal’s lungs artificially. It is not necessary to use a mechanical ventilator provided a suitable anaesthetic breathing system is in use, but using a ventilator will often be more convenient than manually assisting ventilation. A mechanical ventilator will often allow the precise control of the duration of inspiration and expiration, the volume of gas delivered to the lungs and the pressure reached in the airway during inspiration. It is not necessary to administer a NMB agent in order to carry out artificial ventilation but, unless the animal is deeply anaesthetised or is hyperventilated to produce hypocapnia, spontaneous respiratory movements may occur and these may interfere with ventilation and surgery.