Catalase activity was measured in 11 areas of perfused adult rat brain. The hypothalamus and substantia nigra contained the highest activities. The corpus callosum, a white-matter structure, contained intermediate activity. The caudate-putamen and frontal cortex contained the lowest activities. Regional catalase bears some relationship to the reported distribution of microperoxisomes, but considerable activity is present in areas with few microperoxisomes. Catalase may function as one of the systems detoxifying H2O2 formed in CNS amine metabolism.