The specific activities of aminating NADH‐ and deaminating NAD+‐glutamate dehydrogenase (GDH, EC 1.4.1.2) varied considerably in crude extracts of grapevine (Vitis viniferaL. cv. Sultanina) callus and were dependent on the nitrogen source of the culture medium. However, dialysis of the enzyme preparations resulted in a significant decrease in the deaminating GDH specific activity while the aminating activity was not affected. The presence of malate in the crude extract resulted in erroneous overestimation of the NAD+‐GDH activity through the malate dehydrogenase reaction. Thus, in dialysed extracts, the ratio of the NADH‐GDH/NAD+‐GDH specific activities remained relatively constant irrespective of the nitrogen source. In view of this evidence, we now have modified methods for staining both the NADH‐GDH and NAD+‐GDH activities on gels in order to compare the aminating and deaminating activities of each of the 7 GDH isoenzymes. The results from the staining of NADH‐GDH and NAD+‐GDH activity of enzyme preparations from calluses revealed the same isoenzyme profile. Furthermore, separated leaf isoenzymes showed similar activity ratios and kinetic properties. These results may suggest that each one of the 7 isoenzymes have similar in vitro anabolic and cat