For longer procedures, horses are anaesthetized using inhalant anaesthetics, such as isoflurane, sevoflurane and desflurane. As these are important greenhouse gases, who are very resistant to breakdown in the atmosphere, their use should be kept to a minimum. In human medicine, an 'anaesthetic conserving device' is already marketed, which allows a substantially lower use of isoflurane and sevoflurane. Several adjustments are however required before this model can be used in horses. During in vitro experiments with several devices in parallel, the function, resistance, dead space volume, etc. will be measured while using volumes and gas flows which are appropriate for mechanical ventilation in horses. Subsequently, an in vivo test will be performed and a model specifically for horses will be developed, which after new in vitro and in vivo experiments can be used in clinical trials using isoflurane and sevoflurane. This will result in a lower use of inhalant anaesthetics, which not only lowers the cost of anaesthesia, but more importantly also the impact of equine anaesthesia on the environment will be decreased.