Lactate is known to be formed from pyruvate in glycolysis in hypoxic conditions.This study was carried out to investigate the possible change in lactate concentrations of cerebrospinal fluid and plasma in different types of brain tumours. Cerebrospinal fluid and plasma samples were obtained from patients admitted to Dokuz Eyliil University Neurosurgery Department and diagnosed to have brain tumours of various types. Twenty eight cases were divided into iwo distinct categories, as malignant (n=14) and benign (n=14) tumour groups. The control group consisted of fifteen neurologically normal subjects. Cerebrospinal fluid and plasma lactate levels were determined using the enzymatic UV method of Bergmeyer, employing lactate dehydrogenase. The mean cerebrospinal fluid and plasma lactate concentrations were found to be statistically elevated in two distinct type of brain tumour groups, when compared with the controls (p<0.05 and p<0.01, respectively).These findings may reflect a generalized, or localized hypoxy in the tumoural state.