Abstract N-Formylkynurenine and kynurenine are structurally related compounds formed as major products during the biologically important tryptophan oxidation process. Consequently, methods for their simultaneous analytical determination are required. In this investigation, electrochemical studies at mercury electrodes on N-formylkynurenine have been undertaken over a wide pH range and results compared to kynurenine which is reduced in three two-electron steps and for which an analytically sensitive polarographic method is already available. Techniques used to study the mechanism for reduction of N-formylkynurenine include dc, and pulse polarography, linear sweep voltammetry, chronocoulometry and bulk electrolysis with electrochemical and UV spectroscopic monitoring of the reaction. In acid media an overall chemically irreversible, two-electron, two-proton reduction process is observed whereas in alkaline media the process is an irreversible one-electron reduction step. That is, there is one less reduction process than found with kynurenine and the number of electrons transferred in basic media are different. At all pH values the reduction takes place at the γ-carbonyl group. In acid media, the compound formed after reduction is proposed to be γ-( o-formylaminophenyl)-homoserine and reactant adsorption is observed, whereas in alkaline media it is proposed that a dimer is formed from the anion radical without adsorption taking place. Despite their similar reduction potentials the kynurenine and N-formylkynurenine processes are readily resolved by modified forms of differential pulse polarography at all pH values, so that both compounds may be determined simultaneously. Consequently, on standing in solution, additional electrochemical responses observed for N-formylkynurenine can be shown to correspond to reduction of kynurenine formed by hydrolysis of N-formylkynurenine.