Male guinea pigs were fed during 5 weeks with diets differing only in vitamin C content: low (33 mg/kg diet), medium (660 mg/kg), and high (13,200 mg/kg). Heart vitamin C was strongly dependent on dietary vitamin C and heart vitamin E showed a trend to increase as a function of the vitamin C level in the diet. The low vitamin C diet decreased body weight gain, food intake, and heart malondialdehyde without changing lipid peroxidation, whereas the high vitamin C increased oxidized glutathione and glutathione peroxidase and decreased body growth. A tendency to show higher levels of all the first-line antioxidants reduced glutathione, uric acid, superoxide dismutase, catalase, and glutathione peroxidase at extreme (high or low) dietary levels of vitamin C was observed. The guinea pig heart showed capacity for enzymatic but not for non-enzymatic in vitro lipid peroxidation. It is concluded that dietary vitamin C supplementation is able to increase the global antioxidant capacity of the heart tissue.