Angiotensin (Ang) II type 1 (AT1) receptor antagonists are orally active drugs that specifically block the subtype 1 of Ang receptors. In contrast to AT1 receptor antagonists, angiotensin-converting enzyme (ACE) inhibitors block the actions of Ang II incompletely. Furthermore, the bradykinin-potentiating effects of ACE inhibitors may contribute to the mechanism of action of ACE inhibitors. Data in experimental animals suggest that AT1 receptor antagonists decrease the glomerular filtration rate (GFR) to a lesser degree than ACE inhibitors. The greater effect of ACE inhibitors in decreasing glomerular pressure was attenuated with a bradykinin antagonist. In rat models of renal damage with proteinuria, acute reduction of proteinuria was seen with ACE inhibitors but not with AT1 receptor antagonists, whereas long-term reductions of proteinuria were of similar magnitude with both agents. Renal histology after several months revealed that AT1 receptor antagonists and ACE inhibitors were equally renoprotective in various renal damage models. AT1 receptor antagonists, like ACE inhibitors, exhibit a natriuretic effect equal to moderate doses of a thiazide diuretic. In patients with severe volume depletion, use of AT1 receptor antagonists may lead to acute renal failure. Valsartan was tested in a double-blind trial in patients with moderate to severe renal failure and led to a substantial decrease in diastolic and systolic blood pressure, whereas there was no difference from placebo for changes in GFR. Urine protein increased with placebo and decreased with valsartan. The data indicate that valsartan in renal failure patients is effective in lowering blood pressure while leaving renal excretory function unaltered. Whether there is a renoprotective effect can only be shown in long-term trials, which are under way.