FK-506 confers a neuroprotective effect and is thought to extend the time window for thrombolytic treatment of cerebral ischemia. These effects have not been assessed in an embolic stroke model. In addition, clinical studies have raised concern that FK-506 may increase the risk of hemorrhagic transformation by damaging vascular endothelial cells. We investigated whether combined administration of recombinant tissue plasminogen activator (rt-PA) and FK-506 would extend the therapeutic time window without increasing the hemorrhagic transformation in a rat embolic stroke model. Male Sprague-Dawley rats (n=66) were subjected to embolic infarction and assigned into eight groups. Six of the groups were treated with or without FK-506 (0.3 mg/kg) administration at 60 min after embolization, together with and all six groups received systemic rt-PA administration (10 mg/kg) at 60, 90, or 120 min. Two permanent ischemia groups were administered saline either with or without FK-506. Infarct and hemorrhagic volume were assessed at 24 h after embolization. Diffusion-weighted and perfusion-weighted magnetic resonance imaging (MRI) were performed in the groups administered rt-PA at 90 min and a vehicle control group to assess whether FK-506 influenced the effectiveness of MRI in revealing ischemic lesion. FK-506 extended the therapeutic time window for systemic thrombolysis compared to rt-PA alone without increasing the risk for hemorrhage. Combined therapy with FK-506 salvaged some of the MRI, revealing ischemic lesions destined to infarction in the animals treated by rt-PA alone. Single low dose of FK-506 alone did not ameliorate the embolic infarction, but it did prove effective in extending the therapeutic time windows for thrombolysis without increasing the risk of hemorrhagic transformation.