AbstractThe activity of glutamate dehydrogenase (l‐glutamate: NAD oxidoreductase, EC 1.4.1.2.; GDH) of rice plants changes in response to the nitrogen source supplied to the culture solution. The activity of NADH‐GDH(aminating) in roots is rapidly increased by the addition of ammonia, whereas the activity in shoots is much less affected by nitrogen supply. The activity increased with increasing concentration of ammonia at least up to 14.3 mM. In roots GDH activity was found in both the mitochondrial and soluble fractions. The increase of NADH‐GDH activity caused by the ammonia treatment occurs mainly in the latter fraction. The new band with GDH activity was detected on the zymogram of polyacrylamide gel electrophoresis and this inducible enzyme is active with both NAD and NADP. On the other hand, the constitutive enzyme activity active with NAD is also increased by the ammonia treatment. The increase of enzyme activity is prevented by the addition of cycloheximide or chloramphenicol to culture medium. The incorporation of14C‐leucine(U) into GDH proteins was also studied using polyacrylamide gel electrophoresis. Higher radioactivity was found in induced samples than in non‐induced ones. These results show that the increase of GDH activity in roots by ammonia treatment seems to depend onde novoprotein