alpha-Lipoic acid (LA) has demonstrated antioxidant effects in humans and laboratory animals. The objective of this study was to determine whether the effects of LA are similar in horses. Five Thoroughbred geldings were supplemented with 10 mg/kg/d DL-alpha-lipoic acid in a molasses and sweet feed carrier and five received only the carrier as a placebo (CON). Blood samples were obtained at baseline (0 d), after 7 and 14 d of supplementation, and 48 h postsupplementation (16 d). Blood fractions of red and white blood cells (RBC and WBC, respectively) and plasma were analyzed for glutathione (GSH), glutathione peroxidase (GPx) and total plasma lipid hydroperoxides (LPO). An experienced veterinarian observed no adverse clinical effects. Plasma LPO baselines differed between groups (P = 0.002). When covariates were used, there was a decrease over time in the LA group (P = 0.015) and concentrations were lower in the LA group than in the CON group at 7 and 14 d (P = 0.022 and P = 0.0002, respectively). At baseline, GSH concentration was 69 +/- 7 in WBC and 115 +/- 13 mmol/mg protein in the RBC, with no differences resulting from either time or treatment. The GPx activity was 47 +/- 4 and 26 +/- 5 U/g protein at baseline WBC and RBC, respectively, with a lower concentration in the LA group's WBC at 7 (P = 0.019) and 14 d (P = 0.013). The results show that 10 mg/kg LA had no evident adverse effects, and moderately reduced the oxidative stress of horses allowed light activity. These findings encourage studying of LA in horses subjected to strenuous exercise.