Adenylate kinase (AK) activity and glutathione concentration were measured in cerebrospinal fluid (CSF) on three different occasions after the ictus in 16 patients with cerebral infarction. All patients displayed an unambiguous AK activity in their CSF within 48 h after the ictus while glutathione was present in 10 of the 16 patients. The mean AK activity among these patients was reduced by about 60% after 1 week and an additional smaller reduction took place after 3–4 weeks. The glutathione concentration in CSF of these patients showed a similar curve profile with time. The 9 patients who showed clinical signs of improvement had significantly lower values on all occasions for both AK and glutathione than those who did not improve or deteriorated. A corresponding reduction in AK activity with time was not observed in 3 patients with cerebral malignancies. The possibility was excluded that the enhanced AK activity in CSF was the result of an increased transfer of the enzyme from the plasma compartment. Instead a causal relationship was suggested between the appearance of AK in CSF and a disturbed brain cell metabolism. Repeated analyses revealed different curve profiles for ischemic and neoplastic brain lesions. The importance of sampling and handling the CSF material under standardized conditions is emphasize