Activities of adenosine deaminase (ADA), 5'nucleotidase (5'NT), xanthine oxidase (XO), superoxide dismutase (SOD), glutathione pel oxidase (GSH-Px), and catalase (CAT) and levels of thiobarbituric acid reagent substances (TBARS) were measured in 10 cancerous and 10 noncancerous human prostate tissues. Decreased activities of DNA turnover enzymes (ADA and 5'NT), increased activities of GSH-Px and CAT, and unchanged activities of SOD and XO were observed in cancerous prostate tissues compared with those of noncancerous ones. TEARS levels were found to be higher in cancerous tissues than noncancerous ones. In correlation analysis, mostly positive correlations were established between enzyme activities of the cancerous tissues, whereas no meaningful correlations were found between enzyme activities of the noncancerous tissues except for a positive correlation between XO and SOD. The results indicate that the activities of DNA turnover enzymes were reduced, which was possibly an attempt to lower the rate of purine catabolism, and the activities of GSH-Px a,td CAT enzymes were increased, probably in response to increased free radical stress occurring in cancerous prostate tissues. Increased concentrations of TEARS suggested oxidant stress and thus accelerated per oxidative reactions in the cancerous tissues, even though antioxidant defense mechanisms were e activated These findings suggest that enzymatic antioxidant systems of cancerous prostate tissues cannot sufficiently eliminate oxidant factors and prevent cellular peroxidative reactions occurring during the carcinogenic process.