Sodium dithionite reduces folate to 7,8-dihydrofolate at room temperature, but further reduction to the tetrahydro level can be obtained at elevated temperatures (11). Both folate and dihydrofolate form adducts with dithionite (presumably sulfinates), and these adducts appear to be intermediates in the reduction processes. The adduct of dihydrofolate is not converted to tetrahydrofolate at low temperatures, and is thus more amenable to examination. The mechanism of reduction of the folate compounds and of the nicotinamide ring by dithionite involves nucleophilic attack by the dithionite, not a free radical reduction as with p-nitrophenol. The rate of addition of bisulfite to folate and dihydrofolate has been measured by the stopped-flow kinetic method, and the nonenzymic and enzyme-catalyzed addition of nucleophiles to folate compounds is discussed.