Direct surgical attack on intracranial malformations of great complexity may be facilitated by complete cardiocirculatory arrest under hypothermia. To improve the technical aspects of this procedure and to determine the optimum depth of hypothermia, the duration of arrest that can be tolerated, and the role of barbiturate brain protection, we conducted an experimental study on 11 baboons. Arrest lasted for 45 or 90 minutes at a brain temperature of 12 degrees C. The animals were assessed by clinical, neuroradiological and necropsy examination. All 4 animals subjected to 45 minutes arrest made a good clinical recovery while 3 of the 7 subjected to 90 minutes arrest presented thromboembolic complications. Strict heparinization in the last 2 animals solved these problems. The findings of this study may help to make the procedure safer for clinical use.