Several biochemical markers in the cerebrospinal fluid (CSF) of 120 patients with serous meningoencephalitis (SM) of viral origin were compared with those of 74 patients with viral or bacterial infections accompanied by neck stiffness but no CSF abnormality (i.e., meningism). CSF adenylate kinase was higher (P less than 0.025) in SM and correlated with lactate concentration (r = 0.37; P less than 0.01). CSF hypoxanthine was lower (P less than 0.001) in SM, whereas CSF xanthine was similar in the two conditions. The xanthine/hypoxathine ratio correlated with the CSF leukocyte count (r = 0.32; P less than 0.01), and especially with the mononuclear cell count (r = 0.45; P less than 0.001). CSF adenylate kinase correlated with fever in SM (r = 0.28; P less than 0.01). CSF urate and protein displayed a mutual correlation in both conditions (r = 0.26 and P less than 0.05 for SM; r = 0.55 and P less than 0.001 for meningism). These results support the hypothesis of impaired brain cell metabolism, probably of ischemic nature, in viral meningoencephalitis, causing leakage of adenylate kinase into the CSF, where hypoxanthine may be reutilized by mononuclear leukocytes.