The investigation of the intracellular amino acid pool of C. glutamicum during the early phase of growth revealed that glutamate accumulated first and then decreased with increasing culture time; this decrease was found to be independent of osmolarity even though it is known that the glutamate pool increases in a high osmolarity medium. The increase in glutamate concentration could be attributed to de novo synthesis because a synthetic medium was used. This metabolic change could be due to a change in enzymatic activity. The investigation of the changes in the enzymatic activity revealed that 2-oxoglutarate dehydrogenase (ODH) activity was low initially and then increased with culture time. Since ODH catalyzes the conversion of 2-oxoglutarate to succinyl-CoA, the decrease in ODH activity would lead to an increase in 2-oxoglutarate––a direct precursor of the de novo synthesis of glutamate. Therefore, this change in ODH activity corresponded well with the changes in the intracellular glutamate pool described above. Since glutamate serves as an amino group donor for the synthesis of other amino acids, it would be the first to accumulate during the early phase of growth.