Oxidative damage by free radicals has been implicated in the pathogenesis of several diseases. In this study, the antioxidative effect of dietary garlic on rainbow trout was examined. Trout fingerlings were fed on diets containing 10, 20, 30, 40 and 50 g garlic powder kg
1 diet. Serum lipid peroxides and activities of antioxidant enzymes were measured. Serum thiobarbituric acid reactive substances (TBARS) assay showed that garlic consumption that resulted in a significant decrease in lipid peroxidation. The lowest levels of TBARS were observed in fish fed diet supplemented with 30 g kg
1 garlic. However, higher doses of garlic (40 and 50 g kg
1 diet) caused no further reduction in serum TBARS. The results showed a significant increase in superoxide dismutase in all of the garlic-treated groups compared with the control. Ingestion of 10, 20 and 30 g kg
1 dietary garlic resulted in a significant reduction in the catalase activity compared with all but the 10 g kg
1 group. There was no significant difference in glutathione peroxidase activity among the different groups. Serum alanine aminotransferase and aspartate aminotransferase levels increased significantly in trout-fed diets containing 40 and 50 g kg
1 garlic powder. These results suggest that dietary garlic may improve the antioxidant status of rainbow trout. However, undesirable effects of higher doses of garlic should be considered.