The effects of subchronic administration of nerve growth factor (NGF) into the lateral ventricle on catalase and selenium-dependent glutathione-peroxidase (GSH-Px) activity in several areas of the brain in 3-, 12- and 24-month-old rats were studied. NGF given daily (1 microgram for 28 consecutive days) produced in all brain areas studied a significant increase in catalase activity in 12- and 24-month-old rats. The most important finding was a complete restoration in 12- and 24-month-old rats of catalase activity to levels similar to those occurring in young (3-month-old) rats. In addition, NGF produced in comparison to 3-month-old rats and to same age vehicle-treated rats a significant increase in selenium-dependent GSH-Px in all the brain areas studied in 12- and 24-month-old animals, whereas selenium-independent GSH-Px was unaffected. In conclusion, the present results show that long-term administration of NGF into the lateral ventricle significantly increases in old animals the activity of key enzymes involved in the metabolic degradation of hydrogen peroxide.