Effect of supplementation of high levels of folate (200 mg of folate per kg of diet) to vitamin $B_{12}$-deficient rats was studied. Increased total liver folate levels were observed in the vitamin $B_{12}$-deficient rats supplemented with high levels of folate although folate supplementation had no significant effect on normalizing the rate of histidine oxidation and the urinary formiminoglutamate excretion. Supplementation of high levels of folate to vitamin $B_{12}$-deficient rats increased hepatic non-methyl folate levels to normal or supranormal levels and shifted the polyglutamate chain distribution of folate derivatives slightly decreased. However, penta- and hexa-glutamate derivatives of non-methyl folates were found predominantly in the liver of both vitamin $B_{12}$-deficient and vitamin $B_{12}$-supplemented rats in this study. Thus the absolute hepatic levels of total folate or $H_4$PteGlu were not strictly related with the rate of histidine oxidation and the low rate of histidine oxidation in vitamin $B_{12}$-deficient rats supplemented with high levels of folate were not due to the reduced length of polyglutamate chain of folate derivatives. These data suggest that the ratio of $H_4$PteGlu/5-methyl-$H_4$PteGlu may be the more important factor in modifying folate functions and the methyl folate trap may not be circumvented by supplementation of high levels of folate in vitamin $B_{12}$-deficient animals.