Mechanisms underlying differential toleranceto Manganese (Mn) toxicity in perennial ryegrass(Lolium perenne L.) cultivars are poorly understood.We evaluated activity of antioxidative enzymes androot exudation of carboxylates in four ryegrass cultivarssubjected to increasing Mn supply under nutrient solutionconditions. A growth reduction caused by Mntoxicity was smaller in Jumbo and Kingston than Nuiand Aries cultivars. Shoot Mn accumulation varied inthe order Nui > Aries > Kingston > Jumbo. Ascorbateperoxidase and guaiacol peroxidase activities increasedwith Mn excess. Mn-tolerant Jumbo and Kingston hadhigh activity of these enzymes and relatively low lipidperoxidation. Kingston was most tolerant to high tissueMn concentrations and had the highest superoxidedismutase activity. Increased activity of antioxidativeenzymes in Mn-tolerant cultivars could protect theirtissues against oxidative stress triggered by Mn excess.Mn toxicity induced root exudation of carboxylates;oxalate and citrate may decrease Mn availability in therhizosphere, thus enhancing Mn tolerance in ryegrass.