Male mice (4 months old) were fed green tea extract- (GTE; 0.25% w/w) and/or fish oil- (FO; 5% w/w) containing diets with 8 ppm methylmercury (MeHg) chloride for 4 months to investigate the effects of simultaneous intakes on brain functions, antioxidant defenses against MeHg, and MeHg accumulation in tissues. In mouse maze tests, intake of GTE or FO significantly improved the learning ability of MeHg-exposed mice. GTE and FO also significantly decreased liver and brain catalase activities in MeHg-exposed mice. Total Hg concentrations in muscle were significantly lowered by dietary GTE and FO in the MeHg-exposed group, though no remarkable differences were observed in the brain. These data indicate that simultaneous intake of GTE and FO effectively prevents MeHg-mediated oxidative stress and reduces the effects of Hg exposure with fish consumption.