The metabolic, biochemical and molecular events occurring in the different leaf stages along the main axis of tobacco (Nicotiana tabacum) plants grown either on a nitrogen-rich medium, on a medium containing ammonium as sole nitrogen source or on a nitrogen-depleted medium, are presented. This study shows that the highest induction of cytosolic glutamine synthetase (GS1) protein and transcript occurs when nitrogen remobilization is maximal as the result of nitrogen starvation, whereas both glutamate dehydrogenase (GDH) transcript and activity remain at a very low level. In contrast, GDH is highly induced when plants are grown on ammonium as sole nitrogen source, a physiological situation during which leaf protein nitrogen remobilization is limited. It is therefore concluded that GDH does not play a direct role during the process of nitrogen remobilization but is rather induced following a built up of ammonium provided externally or released as the result of protein hydrolysis during natural leaf senescence.