The synthesis of 5-methyl-5,6,7,8-tetrahydropteroly tri-, pena-, and heptaglutamate has been accomplished by reductive methylation of the tetrahydropteroyl oligoglutamate with formaldehyde, followed by purification on DEAE-Sephadex. The corresponding [5-14-C]methyltetrahydropteroyl oligoglutamates were prepared from 14-CH-2-0, and tested as substrates for methionine synthetase (EC 2.1.1.13) ISOLATED FROM BOVINE BRAIN. In all cases, the polyglutamate conjugates were better substrates (lower Km, higher Vmax) than the corresponding monoglutamate forms. In addition, the nonradioactive methyltetrahydropteroyl oligoglutamates inhibited the methylation of homocysteine by methyltetrahydrofolate. This indicates that the monoglutamate and polyglutamates compete for the same enzyme, and established a role for the ubiquitous methyltetrahydropteroyl oligoglutamates in mammalian methionine biosynthesis.