The effects of butyl nitrite, a frequently used recreational drug, on methyl cobalamin and 5-methyl tetrahydrofolate were investigated by using 1H and 31P NMR spectroscopies. While no effect could be observed in organic solvents, strong interactions of butyl nitrite with the methyl cobalamin and 5-methyl tetrahydrofolate were found to occur in water. Butyl nitrite decomposes in water generating H+ and NO2-. The former protonates to give the "base-off" configuration of methyl cobalamin while the CoCH3 bond is cleaved. Similarly, the methyl group at the 5N position and the pyrazine ring of 5-methyl tetrahydrofolate were found to be affected by butyl nitrite. The overall interaction of butyl nitrite with both coenzymes shows displacement of the methyl group and derivatization or destruction of the coenzymes that may lead to deficiencies of both B-12 and/or folates. © 1986.