The role of maize (Zea mays L. inbred line VA35) root mitochondria in nitrogen assimilation was investigated. Maize plants were grown for 2 weeks on a modified Knopp solution containing different forms of nitrogen: 10.9 mM nitrate or 10.9 mM nitrate + 7.2 mM ammonium. Mitochondria were isolated from root tissue and purified on a Percoll gradient. Glutamate dehydrogenase (GDH; EC 1.4.1.2), alanine aminotransferase (GPT; EC 2.6.1.2), NAD + -isocitrate dehydrogenase (NAD + -ICDH; EC 1.1.1.41), succinate dehydrogenase (SDH; EC 1.3.99.1) and malate dehydrogenase (MDH; EC 1.1.1.37) all showed increased specific activities in the case of mitochondria isolated from plants grown in the presence of ammonia. These results indicate the involvement of such mitochondria in ammonia assimilation by synthesizing glutamate in a reductive animation reaction catalyzed by glutamate dehydrogenase and enhanced TCA metabolism. Increased oxygen consumption rates of mitochondria isolated from ammonia grown plants was demonstrated in our experiments, supporting the occurrence of such a metabolic shift. The possibility that higher oxygen consumption rates of both respiratory pathways (phosphorylative and non-phosphorylative) with malate as substrate for oxidation result from stimulation of biosynthetic and catabolic functions of mitochondria from plants grown in the presence of ammonia is discussed.