To investigate brain changes in induced deep core hypothermia (18° C) with or without circulatory arrest, four groups of dogs were subjected to cardiopulmonary bypass (CPB) under the following conditions: (1) differential head perfusion with pulsatile flow and simultaneous circulatory arrest to the rest of the body; (2) differential perfusion to the head with a nonpulsatile flow; (3) total circulatory arrest; and (4) continuous hypothermic perfusion. Parameters analyzed were: (1) blood flow distribution; (2) creatine kinase isoenzyme (CK-BB) elevation in the cerebrospinal fluid (CSF) and in the brain venous return; and (3) microscopy of the brain in animals killed at 30 minutes, 24 and 48 hours, 1 and 2 weeks, and 1 month. Although minor brain tissue flow differences were found at 37° C among the groups, flows equalized at 18° C. A significant seven-fold brain flow increase followed the period of circulatory arrest in Group III. Rise of CK-BB levels occurred in brain venous return but not in CSF in all groups. Microscopic cellular damage appeared in all groups with an equal degree of severity, regardless of the method of hypothermia and perfusion implemented.