It was determined from in vitro experiments that the minimal dose of urokinase required to lyse 10 ml of clotted canine blood within a closed space must exceed 10,000 IU. We empirically doubled this minimum effective dose and tested the in vivo safety of injecting 20,000 IU of urokinase every 12 hours for 4 days into the ventricles of six adult mongrel dogs through an implanted catheter-reservoir system. The animals were monitored carefully for local and systemic bleeding by neurological and clinical examination, hematological tests reflecting systemic fibrinolytic status, serial computed tomography, and postmortem histological examinations of the brain, meninges, and peripheral organs. It was found that this intraventricular dose regimen of urokinase did not cause intracranial hemorrhage even though the dogs had recent brain wounds related to transcerebral ventricular catheterization. Mild activation of systemic fibrinolysis, implying passage of the enzyme from ventricle to blood, occurred 4 to 6 hours after each intraventricular injection, but no systemic hemorrhages were seen. This dose regimen also did not cause acute or chronic inflammatory changes in the brain or meninges and did not disturb cerebrospinal fluid circulation.