During cardiac surgery, as a result of surgical aggression, myocardial ischaemia and cardiopulmonary bypass, the renin-angiotensin-aldosterone mechanism is intensely activated. Our aim is to document whether, in the case of patients undergoing chronic treatment with lisinopril, the non-withdrawal of this inhibitor's administration before cardiac surgery and the administering of a last dose on the day of the operation are associated with coronary haemodynamic alterations. A study was made of 18 patients submitted to myocardial revascularization under extracorporeal circulation and distributed in two groups: group A) without ACE inhibitorsplacebo, group B) with ACE inhibitors (Lisinopril). Coronary blood flow (CBF) was determined by inverted thermodilution via Baim's catheter. Coronary and metabolic haemodynamic values were calculated. Lisinopril had no significant influence on the CBF or on the other above-mentioned values. Therefore, it is not necessary to withdraw ACE inhibitors in cardiac surgery interventions.