This study using pigs was designed to determine the efficacy of delayed subarachnoid plasmin injection in preventing the vasculopathy secondary to experimental subarachnoid hemorrhage. Animals received cisterna magna injections of either plasmin or saline 2, 4, or 6 days after double subarachnoid blood injection. The cerebral blood vessels of all animals were examined histologically for evidence of intimal proliferation and medial necrosis. Angiography was not done. The results indicate a progressive increase in the extent and severity of intimal proliferation the longer plasmin injection is delayed. In contrast, the severity of medial necrosis did not correlate with the delay. We conclude that subarachnoid clot lysis is an effective way to prevent vascular injury from subarachnoid blood and that, the sooner it is done after hemorrhage, the more effective it will be.