We serially measured creatine kinase (CK), lactate dehydrogenase, aspartate aminotransferase (AST) and lactate from the lumbar cerebrospinal fluid in 14 patients with neurologic complications after open heart surgery with cardiopulmonary bypass (CPB). These analyses revealed a correlation between worsening neurologic deficit and the peak CK (r = .87, p less than .001), AST (r = .75, p less than .01), and lactate (r = .93, p less than .001) levels. Lactate increased before enzymes did. In 12 patients without complications, only lactate was significantly (p less than .005) elevated; however, within this group, CK but not lactate could be used to differentiate patients who later developed subtle mental changes. Although CPB appeared to induce metabolic changes in the brain that could possibly disturb function, severe cerebral damage appeared to require additional global or focal anoxic-ischemic factors. Short hypothermia during bypass did not influence CK, but it was falsely elevated after prolonged hypothermic periods. The testing of these enzymes may be a reliable indicator of the degree of brain damage and the prognosis.