Changes in the activity of oxalate oxidase (OxO) and of the concentrations of oxalate and H2O2 were investigated during the ageing of leaf sheaths of ryegrass (Lolium perenne L.) stubble. The accumulation of H2O2 during ageing coincides with the increases of both oxalate level and OxO activity. Western and Northern blot analyses using protein and RNA extracts of the different categories of leaf sheaths suggested that OxO gene expression, as well as Ca-oxalate synthesis, are crucial events of ageing for leaf sheaths. Immunocytochemistry experiments have revealed that OxO, which is an extracellular enzyme, is nearly always present in the parenchymatous cells surrounding the vascular bundles and in the cells of the lower epidermis. Overall, results suggest that in ryegrass that synthesizes both Ca-oxalate and OxO, the production of H2O2 and Ca2+ during ageing of stubble might be involved in the constitutive defences against pathogens, thus allowing the phloem mobilization of nutrient reserves from the leaf sheaths towards elongating leaf bases of ryegrass.