A reduced nicotinamide adenine dinucleotide (NADH)-dependent glutamate synthase has been detected and partially purified from crude extracts of Saccharomyces cerevisiae. The enzyme is specific for NADH, glutamine, and alpha-ketoglutarate (K(m) values of 2.6 muM, 1.0 mM, and 140 muM, respectively) and has a pH optimum between 7.1 and 7.7. The stoichiometry of the reaction has been determined as 2 mol of glutamate synthesized per mol of glutamine consumed. Glutamate synthase can be distinguished from either of the glutamate dehydrogenases of yeast on the basis of its substrate requirements and behavior during agarose gel and ion exchange chromatography. Variations in the specific activity of glutamate synthase, which occur in response to changes in the growth medium, are similar in character to those observed with the nicotinamide adenine dinucleotide phosphate-dependent (anabolic) glutamate dehydrogenase.