Each nucleus ambiguus of the rabbit contains about 1,500 cells and is approximately 4 to 5 mm long. The author confirms the previously described division of the nucleus into a rostral compact formation, 1.6 mm long, and a caudal diffuse formation occupying the remainder of the nucleus. He further subdivides the compact formation into a lateral part, the principal column, and a medial part, the medial column. These divisions and the cytological differences between the cells in different divisions are illustrated. The cells in the medial column are more similar in appearance to those of the diffuse formation, with which they probably form a continuous column, than to those of the principal column, which are smaller, and have less distinct Nissl granules.