The biosynthesis and degradation of NAD-linked glutamate dehydrogenase isozymes of red blood cells from phenylhydrazine-treated rabbits have been studied. The results were as follows. The normal red blood cells contained about 4.0 units per ml of red blood cells. The cytosolic isozyme activity was increased significantly after phenylhydrazine treatment and reached a maximum value at the 5th day, that is about 8.6 fold as compared with initial value. The mitochondrial isozyme activity was detected at the 2nd to 3rd day after phenylhydrazine treatment and also attained the peak at the 5th day. Accordingly the increase in total glutamate dehydrogenase activity during the induction of reticulocytosis appeared to be primarily due to the increase in the cytosolic isozyme ($K_s=0.1161$; $K_d=0.0043$; $t_{1/2}=161.16$ hours) and partly due to the increase in the activity of mitochondrial isozyme during reticulocytosis ($K_s=0.0675$; $K_d=0.0090$; $t_{1/2}=77.00$ hours). The rate constants following recovery from phenylhydrazine treatment were: $K_s=0.0916$; $K_d=0.0229$; $t_{1/2}=30.26$ hours for cytosolic isozyme and $K_s=0.0160$; $K_d=0.032$; $t_{1/2}=21.66$ hours for mitochondrial isozyme.