Intravenous administration of tissue plasminogen activator immediately after the injection of numerous small blood clots into the carotid circulation in rabbit embolic stroke model animals caused a significant reduction in neurological damage. In vitro studies indicate that tissue plasminogen activator produced substantial lysis of clots at concentrations comparable to those expected in vivo, suggesting that this may be the mechanism of action of this drug. Drug-induced hemorrhages were not demonstrable. Tissue plasminogen activator may be of value for the immediate treatment of embolic stroke.