We examined the antioxidant and pro-oxidant activities of p-hydroxybenzyl alcohol (p-HBA), the major component of Gastrodia elata Bl. (GE), and compared them with those of vanillin, which is also a component of GE and a known antioxidant. Both p-HBA and vanillin are powerful scavengers of 1,1-diphenyl-2-picryl hydrozyl, superoxide and hydroxyl radicals. Like vanillin, p-HBA also inhibits iron-dependent lipid peroxidation in rat brain homogenate, microsomes and mitochondria. In addition, p-HBA and vanillin in a dose-dependent fashion inhibit Fe(II)-H2O2-induced damage to benzoate, deoxyribose, glutamic acid, 2-aminobutyric acid and methionine, as well as benzoate hydroxylation. Vanillin has a pro-oxidant effect on Fe(III)-superoxide-induced damage to benzoate, deoxyribose, amino acids and benzoate hydroxylation, whereas p-HBA shows no pro-oxidant activity in the system. Vanillin and p-HBA stimulate bleomycin-iron-dependent damage to DNA only at very high concentrations. These findings suggest that the antioxidant effect of GE extract in the rat brain may result from the antioxidant actions of p-HBA and other phenolic compounds such as vanillin at the cellular and molecular level in brain.