Plasma-, erythrocyte- and liver Se-dependent glutathione peroxidase (GPx) activity, ascorbic acid (AA) concentration, and erythrocyte and liver Se levels were analyzed in groups of guinea pigs fed a Torula yeast-based semipurified diet supplemented with either 0 (-AA), 200 (+AA), or 400 (++AA) mg of AA/kg of diet and either 0.05 (+Se) or 0.2 (++Se) ppm of Se for 24 days. Plasma, erythrocyte, and liver AA concentration were directly related to the levels of AA fed. There were 21.2 ± 2.7% and 51.2 ± 4.5% (p < 0.0001) increases in erythrocyte and liver Se, respectively, in ++Se guinea pigs compared to +Se guinea pigs. Dietary AA had no effect on erythrocyte or liver Se levels. Plasma, erythrocyte, and liver GPx specific activities were significantly (p < 0.001) increased in ++Se guinea pigs compared to +Se guinea pigs. In addition, liver, plasma, and erythrocyte GPx activities were directly related to the level of AA fed. These data indicate that dietary AA has no influence on tissue levels of Se but increases plasma, liver, and erythrocyte GPx activities in the guinea pig.