Angiotensin-converting enzyme (ACE) inhibition significantly decreases plasminogen activator inhibitor-1 (PAI-1) without altering tissue plasminogen activator (tPA) during activation of the renin-angiotensin-aldosterone system in humans. Because ACE inhibitors and angiotensin II type 1 (AT(1)) receptor antagonists differ in their effects on angiotensin II formation and bradykinin degradation, the present study compared the effect of equivalent hypotensive doses of an ACE inhibitor and AT(1) antagonist on fibrinolytic balance. Plasma PAI-1 antigen, tPA antigen, plasma renin activity, and aldosterone were measured in 25 normotensive subjects (19 white, 6 black; 14 men, 11 women; mean age 38.5+/-1.8 years; mean body mass index 25.3+/-0.7 kg/m(2)) during low salt intake alone (10 mmol Na/d), low salt intake + quinapril (40 mg PO bid), and low salt intake + losartan (50 mg PO bid). Compared with low salt alone (systolic blood pressure [BP] 118.8+/-2.2 mm Hg), both quinapril (106.3+/-2.5 mm Hg, P<0.001) and losartan (105.4+/-2. 8 mm Hg, P<0.001) reduced BP. No statistical difference was found between quinapril and losartan in their BP lowering effect. Losartan (P=0.009), but not quinapril, lowered heart rate. Both drugs significantly lowered aldosterone (P<0.001 versus low salt alone for each); however, this effect was significantly greater for quinapril than for losartan (P<0.001 for quinapril versus losartan). Treatment with quinapril, but not with losartan, was associated with a decrease in both PAI-1 antigen (P=0.03) and activity (P=0.018). PAI-1 activity was lower during treatment with quinapril than with losartan (P=0.015). The average PAI-1 antigen concentration was 13. 0+/-2.0 ng/mL during low salt alone, 10.5+/-1.6 ng/mL during quinapril treatment, and 12.3+/-2.1 ng/mL during losartan treatment. In contrast, plasma tPA antigen concentrations were reduced during treatment with losartan (P=0.03) but not with quinapril. This study provides the first evidence that ACE inhibitors and AT(1) antagonists differ in their effects on fibrinolytic balance under conditions of activation of the renin-angiotensin-aldosterone system. Further studies are needed to address the mechanism for the contrasting effects of these 2 classes of drugs on fibrinolysis and to define the clinical significance of these differences.