This review summarizes physiology of circulating and local reninangiotensin system (RAS), enzymatic properties and mechanism of action of angiotensin I converting enzyme inhibitors (ACEIs) on RAS, and implications of ACEIs in anesthetic management of patients treated with these drugs. ACEIs, through their effect on RAS, may improve cardiovascular functions, pulmonary dynamics, and body fluid homeostasis. Thus, ACEIs have become an integral part of management of patients with hypertension, congestive heart failure (CHF) and chronic renal disease. ACEIs, due to differences in their chemical structure, exert different pharmacological actions and can have protective or occasional damaging effects on different organs. The anesthesiologists are commonly involved in the management of patients treated with ACEIs. Thus, the role of ACEIs and their possible interaction with anesthetic agents must be an integral part of clinical decision-making during anesthesia Hemodynamic variation during anesthesia is mainly related to specific effects of anesthetic agents on sympathetic nervous system. Those with preoperative fasting, volume depletion and extended sympathetic blockade can have reduced vascular capacitance resulting in decreased venous return, reduced cardiac output and severe arterial hypotension. Angiotensin II (ANG2) a potent vasoconstrictor may counterbalance such hypotensive effect. During ACE inhibition ANG2 cannot counterbalance this hypotension. Thus, induction of anesthesia may cause severe hypotension in hypovolemic patients specifically in those receiving diuretics as a complement to ACEIs. Recent advances in RAS and the pharmacology of ACEIs have identified some predisposing factors and risks associated with anesthesia in patients treated with ACEIs. Practitioners should be vigilant, and readily have vasopressors, necessary fluids and other resuscitative measures for treatment of unexpected hemodynamic instability during anesthesia and surgery.