Excitotoxic lesions induced by systemic injection of kainic acid, resulted in 2-3-fold increase of xanthine dehydrogenase and xanthine oxidase activities in the rat olfactory cortex 48-72 h after drug administration. A significant increase of the xanthine oxidase/dehydrogenase ratio was also observed at 4 and 48 h post-injection. No similar changes were noticed in the hippocampus. The enhancement of enzyme activity seems to be primarily a consequence of the altered cell composition in damaged area. Free radicals produced by the increased oxygen-dependent form of the enzyme could in turn aggravate the excitotoxic brain injury.