The aim of the present study is to evaluate the oxidative damage in rats of different ages. Weaned rats of 25 g and adults of 300 g were used in groups of 6, a single i.p. dose of morphine sulfate of 3, 6 or 12 mg/kg was administered. All animals were sacrificed to measure GSH and 5-HT levels in brain by liquid chromatography, as well as Na(+), K(+)-ATPase and total ATPase enzymatic activity. 5-HT levels decreased significantly (p < 0.05) in adult animals that received 3 and 6 mg morphine. Na(+), K(+)-ATPase activity increased significantly (p < 0.05) in all groups of weaned animals. In adult animals, Na(+), K(+)-ATPase and total ATPase partially diminished. GSH levels diminished significantly (p < 0.05) both in weaned and in adult groups. The results indicate age-induced changes in cellular regulation and biochemical responses to oxidative stress induced by morphine.