We analyzed the effect of marijuana and nalbuphine on levels of 5-hydroxyindol acetic acid and lipid peroxidation in rat brain. Single and repeated dosages of 250 mg/kg marijuana extract or 10 mg/kg nalbuphine were administered to male and female Wistar rats. Animals were sacrificed and brains were obtained to measure the content of 5-hydroxyindol acetic acid, reduced glutathione, thiobarbituric acid reactive substances, total ATPase and Na+/K+ ATPase activities. There was an increase in thiobarbituric acid reactive substances, total ATPase and Na+/K+ ATPase activity in the animals that received a single dose of marijuana and nalbuphine (p=0.001), with a notable decrease in glutathione and 5-hydroxyindol acetic acid levels (p=0.001). Both marijuana and nalbuphine increased levels of oxidative damage biomarkers in rat brain and decreased glutathione and 5-hydroxyindol acetic acid levels which could provoke changes in cellular and biochemical regulations and serotonergic activity in either male or female rats.