Glutamic dehydrogenase activity proved to be especially high in the cortexphloem region of the young lateral roots. Extracts of other organs and tissues were much less active. In all tissues examined, glutamic dehydrogenase appeared to be localized in the mitochondrial fractions. The relative amounts of the mitochondria in the cells of the various tissues probably determine the glutamic dehydrogenase content of these tissues. a-Ketoglutarate concentrations were also highest in the youngest roots and much lower in main root and stem tissues. It is suggested that the uptake of ammonium from the nutrient solution and its assimilation to glutamic acid are two aspects of the same chemical reaction.