Chronic exposure to stress alters the prooxidant-antioxidant balance, which might lead to the development of various human pathological states. In order to explain the role of antioxidant response in stress-induced injury, we examined the effects of two types of acute stress, as well as combined effects of chronic and acute stress on manganese-superoxide dismutase, copper,zinc-superoxide dismutase, and catalase activities in rat brain hippocampus. Our results show that chronic stress induces an increase in oxidative enzyme activities and that adaptation to chronic stress might alter hippocampal antioxidant mechanisms' response to acute stress.