The angiotensin II (A II) type 1 receptor (AT1) antagonists represent a new pharmacologic class of drugs. These drugs antagonize A II induced biologic actions. Initial clinical trials suggest that these drugs are effective in the treatment of essential hypertension and hypertensive patients with renal disease. Losartan becomes the first potent, orally active and longacting nonpeptide A II receptor antagonist to be used in humans. Antihypertensive efficacy appears to be dependent on an activation of renin-angiotensin system, since bilateral nephrectomy and/or volume expansion abolishes the blood pressure lowering effect. Blood pressure is lowered without a significant change in heart rate or cardiac output. In hypertensive patient with renal disease, losartan has been reported to have no clinically important effect on glomerular filtration rate. To date, no specific AT1 antagonists' side effect has been reported. The potential widespread use of this new class of antihypertensive drugs will depend on whether AT1 receptor antagonists can be differentiated clinically from ACE inhibitors, and if such differentiation has clinically significant benefits.