The long-term administration of ACE inhibitors to selected patients with left ventricular dysfunction appears to reduce the incidence of recurrent myocardial infarction (MI) and unstable angina pectoris. The mechanisms responsible for the reduction in ischemic events are unknown, but likely candidates include effects on the atherosclerotic process, thrombosis, and/or vascular tone. The effects of ACE inhibitor therapy with ramipril on plasma fibrinolytic variables were assessed in 120 subjects participating in the Healing and Early Afterload Reduction Therapy (HEART) study, a double-blind, placebo-controlled trial of acute anterior MI patients who were randomly assigned within 24 hours of the onset of symptoms to receive low-dose ramipril (0.625 mg daily), full-dose ramipril (1.25 mg titrated to 10 mg/d), or placebo for 14 days. Plasma levels of plasminogen activator inhibitor-1 (PAI-1) activity and PAI-1 antigen and tissue plasminogen activator (TPA) antigen were measured before randomization and on day 14. Clinical characteristics of the three study groups were similar, as were the prerandomization plasma levels of PAI-1 antigen, PAI-1 activity, and TPA antigen. Compared with the placebo group, PAI-1 antigen levels were 44% lower (P=.004) at day 14 in the ramipril-treated patients, and PAI-1 activity levels were 22% lower (P=.02). In contrast, plasma TPA levels were not significantly different between the placebo-treated and ramipril-treated groups. Treatment with ramipril has a significant impact on plasma fibrinolytic variables during the recovery phase after acute MI. The renin-angiotensin system appears to play an important role in the regulation of vascular fibrinolysis, and interruption of this regulatory pathway may contribute to the clinical benefits of ACE inhibitors.