Glutamate dehydrogenase becomes density labeled through the incorporation of deuterium and 15N when detached oat leaves (Avena sativa var. Fulghum) are incubated in the presence of ammonia. The enzyme has been isolated by means of DEAE-cellulose chromatography, ammonium sulfate precipitation, isopycnic equilibrium centrifugation, and disc electrophoresis from leaves fed l-methionine-35S. Radioactivity is incorporated into isozyme 1 of glutamate dehydrogenase, whereas isozyme 2, detected only in the absence of ammonia, has not been labeled. Cycloheximide, chloramphenicol, puromycin, and 6-methyl purine inhibit the elevation of glutamate dehydrogenase by ammonia. It is suggested that the increase in glutamate dehydrogenase activity is due to de novo synthesis of isozyme 1.