Many factors, including exogenous carbon metabolites influence the expression of enzymes involved in nitrogen metabolism, but little is known about the effect of low temperature on the expression of those enzymes. To investigate that rice (Oryza sativa L.) seedlings grown at the optimal temperature (30 8C) were subjected to low temperature (20 8C) stress, and several key enzymes involved in ammonium assimilation and carbon metabolism, including glutamine synthetase (GS; EC 6.3.1.2), NADP-dependent isocitrate dehydrogenase (NADP-ICDH; EC 1.1.1.42), and NAD(H)-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) from rice roots were assessed. At low temperature, GS and ICDH activities were increased by 80 and 75%, respectively; thus, a positive parallel correlation was observed between the two enzymes. Native-PAGE analysis, together with activity staining and Western blot assays, showed that both GSrb activity and GSrb protein level were enhanced under low temperature. Meanwhile, NADH-GDH and NAD-GDH activity were both reduced, though to different extents. Biomass, NH4+, pH, and proline were measured as well. At low temperature, ammonium absorption was stimulated by the elevation of GS activity. Proline content was increased two-fold by low temperature, and this accumulation was in good agreement with the induction of GS activity. Similar observations were made in the rice roots fed with sucrose, indicating that at least partially low temperature has the similar effect as carbon compounds on the modulation of nitrogen metabolism. The possibility of the regulation of this metabolic pathway by 2-oxoglurate (2-OG) is discussed.