Early changes of the activity of enzymes such as creatine kinase in the cerebrospinal fluid (CSF) or serum are often investigated after head injuries to assess the extent of brain damage and establish a reliable prognosis. The purpose of the present study was to determine levels of creatine kinase isoenzyme CK-BB in the CSF of rats after experimental head injuries. External head injuries of different severity were inflicted on rats, immediately after which CSF was collected for isoenzyme activity determination. It was found that the levels of CK-BB were significantly elevated immediately after the head injury and that the greater the degree of external cranial injury inflicted, the higher the isoenzyme activity was. The results seem to provide evidence that CK-BB activity is an early indicator of brain damage and that its level may reflect the extent of cerebral damage involved.