Abstract The objective of this study was to evaluate whether the oxidative stress caused by aluminum (Al) toxicity is a symptom that can trigger root growth inhibition in oat genotype seedlings. Oat seedlings were grown in a nutrient solution (pH4.0) with 0 and 370μM Al. At 12, 24, and 36h after Al addition, growth (root length) and biochemical parameters (catalase — CAT, ascorbate peroxidase — APX, and superoxide dismutase — SOD activities, lipid peroxidation, ascorbic acid (ASA) and non-protein thiol group (NPSH) concentration) were determined. The aluminum content was measured in oat seedlings. Regardless of the exposure time, root of the tolerant genotype grew normally with any Al treatments. Al supply caused lipid peroxidation only in the Al-sensitive genotype in roots and shoots (at 12, 24, and 36h). In sensitive genotype seedlings, CAT, APX, and SOD were activated only at 24 or 36h. In tolerant and intermediate genotypes, CAT, APX, and SOD were activated at 12, 24, and 36h. Data for root growth and lipid peroxidation suggested that lipid peroxidation in the sensitive genotype may be an effect of Al toxicity on root growth. Therefore, the tolerant, intermediate, and sensitive genotypes differ in the expression of the amount, type of antioxidants, and speed of activation of antioxidant system, suggesting a varying capacity of these genotypes to deal with oxidative stress, which resulted in varying sensitivity and tolerance to Al.