Prolonged operation or a current need to improve intraoperative conditions, while anaesthesia is still on, may require applying controlled ventilation (CV). The purpose of this study was to evaluate cardiovascular effects of such a decision. Seven halothane-anaesthetized horses were the subject of our investigation, at first kept spontaneous ventilation (SV), then they were connected to respirator. After 30' CV, SV was restored and anaesthesia was continued. In those three stages of experiment haemodynamic parameters and gasometry were measured. It was found that during CV period both optimal gas exchange as well as the lowering of cardiac output actually took place. The authors suggest that equalization of circulation blood volume, which leads to the ventricular proload increase, will enable the veterinarians to apply CV not only because of life-rescular indications, but also to assure safe anaesthesia course.