The renin angiotensin system is activated during anaesthesia and operation, and the degree to which this activation takes place depends on the hydration and circulation of the patient. The anaesthetic agent used is also important for the activation. Treatment with ACE-inhibitors blunts or abolishes the sympathetic response to anaesthesia/operation. ACEI treatment reduces the need for other hypotensive agents in controlled hypotensive anaesthesia. Some studies suggest that ACEI-treatment should be continued peroperatively on account of its stabilising effect on the circulation to avoid sudden fluctuations in blood pressure and organ perfusion. On the other hand measurements of the cerebral blood flow under anaesthesia in patients treated with ACEI have shown low values in a few patients and this led the authors to recommend discontinuation before anaesthesia. We conclude that controlled clinical trials to evaluate the haemodynamic and neurohumoral consequences of the interaction between ACEI and anaesthetics are required.