Creatine kinase (CK) isoenzymes were determined on 15 patients undergoing open heart surgery with bypass. The study was performed to examine the relationship between the hypothermia under which the surgery is conducted and the appearance of CK isoenzymes in the serum both during and after surgery. All patients showed dramatic rises in total CK activity commencing during surgery. Myocardial CK (CK-MB or CK-2) was seen in fourteen patients and brain CK (CK-BB or CK-1) was seen in ten patients. Peak activities of CK-BB did not coincide with peak activities of CK-MB. The serum elevations of CK-BB in these patients appear to arise from a mechanism different from that responsible for the elevations of CK-MB, and it is assumed that the former is due to intermittent disruptions in the perfusion and/or oxygenation of tissues rich in CK-BB. CK-MB elevations appear to be due directly to the surgical intervention. Hypothermia alone does not in itself appear to be solely responsible for elevations of CK-BB although it can not be completely excluded from playing some role in its production.