Abstract: Objective To observe the influence of various methods of cerebral protection during deep hypothermic circulatory arrest (DHCA ) on S-100 protein. Methods Eighteen dogs were randomly and equally divided into three groups: the deep hypothermic circulatory arrest (DHCA group ) , the DHCA with retrograde cerebral perfusion (DHCA + RCP group ) , and the DHCA with intermittent antegrade cerebral perfusion (DHCA + IACP group ). Upon interruption of cardiopulmonary bypass (CPB) , the nasopharyngeal temperature was slowly lowered to 18℃, before CPB was discontinued for 90 minutes, after 90 minutes, CPB was re-established and the body temperature was gradually restored to 36℃, then CPB was terminated. Before the circulatory arrest, 45min, 90min after the circulatory arrest and 15min, 30min after re-established of CPB, blood samples were drawn from the jugular veins fo r assay of S-100 protein. Upon completion of surgery, the dogs was sacrificed and the hippocampus was removed from the brain, properly processed for examination by transmission electron microscope for changes in the ultrastructure of the brain and nerve cells. Results There was no significant difference in the content of S-100 protein before circulatory arrest among all three groups (P gt; 0.05). After circulatory arrest, DHCA and DHCA +RCP group showed an significant increase in the content of S-100 protein (P lt; 0.01). There was no significant difference in the content of S-100 protein after circulatory arrest in DHCA + IACP group. Conclusion Cerebral ischemic injuries would occur if the period of DHCA is prolonged. RCP during DHCA would provide protection for the brain to some extent, but it is more likely to cause dropsy in the brain and nerve cells. On the other hand IACP during DHCA appears to provide better brain protection.