Superoxide dismutase in nervous system protects readily oxidizable compounds such as catecholamines against toxic effects of oxygen. We investigated superoxide dismutase activity during development in 5 brain regions selected for a wide range of catecholamine concentration and turnover: cerebellum, neocortex, striatum, hypothalamus and medulla-pons. The cytosolic and the particulate enzyme were measured from birth to 6 months of age. In cerebellum the cytosolic enzyme shows considerable activity on the first postnatal days; the particulate enzyme is less active, both reach a maximum at 3 months. In cortex and striatum both activities were low during the postnatal days and reach a plateau at 3 months. In hypothalamus both activities are higher during the postnatal days and reach a maximum at 3 months. In medulla-pons the values are 2 times higher than in all other regions; the cytosolic enzyme reaches a maximum at 2 months whereas the particulate enzyme reaches a plateau at 3 months. Thus our results show an increase of superoxide dismutase activity during development in all brain regions; the highest activities were found in regions with high catecholamine content.