Through the multiple actions of angiotensin II (AII), the renin-angiotensin system (RAS) participates in cardiovascular homeostasis. Angiotensin II acts by binding to specific membrane-bound receptors, which are coupled to one of several signal transduction pathways. These AII receptors exhibit heterogeneity, represented by AT1 and AT2 receptor subtypes. The AT1 receptor mediates the major cardiovascular action of the RAS. This receptor has been cloned from multiple species, disclosing features consistent with a transmembrane, G-protein-linked receptor. Further AII receptor heterogeneity is evident by the cloning of isotypes of the AT1 receptor. Blocking the interaction of AII with its receptor is the most direct site to inhibit the actions of the RAS. Many AII receptor antagonists, including peptide analogs of AII and antibodies directed against AII, possess unfavorable properties that have limited their clinical utility. The discovery and further development of imidazole compounds with AII antagonist properties and favorable characteristics, however, has promise for clinical utility. The leader in this field is a selective AT1 receptor antagonist losartan (previously known as DuP 753 or MK-954). Losartan was demonstrated to be an effective antagonist of many AII-induced actions and an effective antihypertensive agent in many animal models of hypertension (HTN). Losartan also demonstrated secondary benefits in preventing stroke, treating congestive heart failure (CHF), and delaying the progression of renal disease in animal models. Clinical studies confirm the AII antagonist action of losartan and suggest that losartan will be effective in the treatment of essential HTN. AII antagonism is likely to provide useful treatment in essential HTN and CHF, conditions in which the RAS is known to play a major role. The utility of AII antagonism may extend beyond that of HTN and CHF, as suggested by the potential usefulness of angiotensin-converting enzyme (ACE) inhibition in the treatment or prevention of many other diseases. The key advantage AII antagonists provide over ACE inhibitors is that they may avoid unwanted side effects, related to bradykinin potentiation with the latter drugs. The AII antagonists will help determine the role of the RAS in physiologic regulation and in the pathophysiology of various disease states.