Systemic fibrinolytic therapy for acute stroke is no longer recommended because of resulting systemic fibrinolysis and the risk of intracerebral hemorrhage. Human tissue-type plasminogen activator (TPA) is a native enzyme that converts plasminogen to plasmin with subsequent clot lysis. The affinity for plasminogen is increased several-fold when the substrate is bound to fibrin. At appropriate dosage, "clot-specific" thrombolysis may be achieved at the surface of the thrombus without creating systemic fibrinolysis. The authors designed a study to evaluate the effect of intravenous TPA administered 2 hours after acute thromboembolic stroke in rats. This time course was chosen to simulate an analogous clinical situation. Middle cerebral artery embolic stroke was caused by intracarotid injection of 0.025 cc of human blood clot in 16 rats. Regional cerebral blood flow, measured by the hydrogen clearance technique, and electroencephalographic (EEG) recordings were obtained every 30 minutes for 5 hours after thromboembolism. Eight rats received a 1-hour infusion of intravenous TPA (1.5 mg/kg) 2 hours after injection of emboli. Ipsilateral blood flow increased significantly within 30 minutes after intravenous TPA and reached preembolic levels within 60 minutes. Blood flow did not improve in the eight control rats throughout the experiment. Power spectral analysis of the EEG recordings showed improvement in the treated group compared to the control group. Postmortem angiography revealed proximal middle cerebral artery occlusion in control animals and patent middle cerebral arteries in TPA-treated animals. Serum fibrinogen and fibrin split products were unchanged in both groups, indicating the absence of systemic fibrinolysis. There were no intracerebral hemorrhages. It is concluded that, in this rat model, TPA increases blood flow with subsequent improvement in the EEG recording after thromboembolic stroke without evidence of systemic fibrinolysis. Intravenous TPA may be useful in the treatment of acute stroke in man.