Free radical damage is implicated in the course of many diseases, including age-related dementias. Oxidative deamination of primary monoamino oxidase (MAO) produces NH3 and H2O2 with established or potential toxicity. MAO activity is increased in aged rat brain and significantly lowered by chronic hydergine (codergocrine mesylate, Sandoz) treatment. The aim of this study was to investigate the effects of hydergine on enzymatic antioxidant defense systems. Hydergine or vehicle was administered systemically to young (3 months) and aged (18 months) Sprague-Dawley rats for 20 days and 24 h after the termination of the treatment, superoxide dismutase (SOD) and catalase (CAT) activities were determined in some brain regions. SOD and CAT activities were higher in the aged animals and were further increased with hydergine treatment. The increase in SOD levels caused by hydergine treatment in the aged animals were the most prominent in the hippocampus and in the corpus striatum. There was no region-specific effect of hydergine treatment on CAT levels in aged animals. The possible causal relationship between increased MAO activity, a generator of free radicals, and increased antioxidant defense in aging brain require further investigation. Decreasing MAO levels and supporting the antioxidant enzymes may underlie the efficacy of hydergine in the treatment of age related cognitive decline.