Diets high in methionine lead to elevation of plasma homocysteine levels which are possibly linked to neurodegenerative diseases and oxidative stress. In the present study, we investigated the effects of methionine-enriched diet on antioxidant defences, on rat spontaneous behaviour and on the ability to sustain long-term potentiation in the dentate gyrus (DG). Sprague-Dawley rats were fed either a standard laboratory diet or a methionine enriched-diet (1% or 5% methionine in drinking water) for 8 weeks. After the 8 weeks, the animals were tested for spontaneous motor activity and habituation in an open field maze, for anxiety-like behaviour in an elevated plus maze and for the ability to sustain long-term potentiation (LTP) induced in the dentate gyrus under urethane anaesthesia. The brains were then removed and histochemically stained for superoxide dismutase (SOD) activity. Rats fed on 5% methionine significantly reduced total distance travelled during the open field test and exhibited no habituation with respect to the other two groups. Rats fed on 5% methionine also showed a significant increase of the anxiety level. Moreover, in this group, the ability to induce LTP in DG was impaired. SOD activity was significantly increased in the cerebral cortex of the rats fed on 1% and 5% methionine with respect to the control group. In conclusion, 5% methionine in drinking water led to evident impairment of locomotor skills and of synaptic plasticity. SOD activity in the cortex was increased in both the groups fed on 1% and 5% methionine, thus suggesting that metabolic adjustments, triggered by the methionine-enriched diet, are likely mediated by reactive oxygen species.