The author estimated the effect of ACTH administration on the transaminase activity in the rat brain and liver. The activities determined were aspartate-glutamate (AsGT), alanine-glutamate (AlGT) and γ-aminobutyrate-glutamate (γAGT) transaminases. The AsGT activities in the brain and liver of the normal rat increased by the administration of ACTH. When ACTH was administered to the hypophysectomized rat, the previously increased activities of the AsGT in the brain and liver decreased to the normal values. The ACTH administration was found noneffective on the AlGT and γAGT activities in the brain and liver of the normal and hypophysectomized rat. The author described some discussion about the difference of the effect of ACTH on the activities of the various transaminases.