The reduction of yeast glutathione reductase by reduced nicotinamide hypoxanthine dinucleotide phosphate (NHxDPH) has been examined by stopped-flow kinetic methods. Like reduced glutathione or NADPH, this pyridine nucleotide generates the catalytically active two-electron reduced form of the enzyme. This reductive half-reaction with NHxDPH has only one detectable kinetic step which shows saturation kinetics (Kd = 76 microM), and has a limiting rate constant of 56 s-1. Comparison of stopped-flow and steady-state turnover data indicates that the reductive half-reaction is rate-limiting in the overall catalytic reaction. No evidence was found for a preequilibrium charge-transfer complex between NHxDPH and the active site FAD, like that seen when NADPH is the electron donor.