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Background and aims: Aluminium (Al) toxicity limits pasture production in acidic soils, and there is evidence that antioxidant enzymes such as superoxide dismutase (SOD) play a key role against Al-induced oxidative stress. We have investigated (i) the relationship between Al-toxicity and oxidative damage as well as the time-course of SOD activity and (ii) the expression profiles of two SOD isoforms in the roots of two ryegrass cultivars. Methods: Jumbo (Al-sensitive) and Nui (Al-semi-tolerant) ryegrass cultivars were cultured hydroponically with 0 or 0.2 mM Al. Roots were colleted during 48 h to determine Al uptake, lipid peroxidation, SOD activity and the expression of LpCu/Zn-SOD and LpFe-SOD isolated genes. Results: LpCu/Zn-SOD and LpFe-SOD were classified phylogenetically as putative mitochondrial and plastidial SODs, respectively. Although Al accumulation did not differ between the two cultivars, lipid peroxidation increased 2.2-fold in Jumbo but only scarcely in Nui by the end of the time-course. An early response in terms of total SOD activity and differential expression of LpCu/Zn-SOD and LpFe-SOD genes occurred in both cultivars after Al treatment. Conclusions: SOD isoforms were up-regulated over a longer period in Nui compared with Jumbo, leading finally to a greater protection in roots against the short term Al-toxicity. [ABSTRACT FROM AUTHOR]