Effect of supplementation of high levels of folate (200 ㎎ of folate per ㎏ 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 H₄PteGlu 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 H₄PteGlu/5-methyl-H₄PteGlu may be the more important factor in modifying folate functions and the methylfolate trap may not be circumvented by supplementation of high levels of folate in vitamin B_(12)-deficient animals.