Nine adult rabbits, perfused with 10% formalin, were used in this experiment. Five of them were mounted on the stereotaxic apparatus according to the Sawyer's atlas. Brains were removed and made serial sections including thalamus and hippocampus. The neocortices of the other four rabbits were removed and the hippocampal formations were exposed. Serial sections of the hippocampus were made and stained alternately with Nissl's method and the silver impregnation method of Glees. The results could be summarized as follows.1. The hippocampus measured 28.00-36.00 mm in length, 6.00-8.50 mm in breadth and 2.68-3.39 mm in thickness. The total numbers of the pyramidal cells (CA_1-CA_4), the granular cells in the dentate gyrus and the basket cells were 7.36?10~6, 4.93?10~6, 2.40?10~5 respectively. The ratio of the total numbers among the pyramidal, granular and the basket cells was 30:20:1 approximately. The total number of the CA_1 pyramidal neurones was 4.11?10~6, and that of the CA_3 was 2.48?10~6. The ratio of the numbers of the CA_1 and CA_3 neurones was 1.66:1.2. The arrangements of the granular cells in the dentate gyrus and the apical dentrites of the CA_1 neurones were rather regular and possessed a typical array-like structure.3. The total numbers of the fibers in the column fornix (FN)and the mamillothalamic tract (MT) were 83700 and 58740 respectively. MT:FN=1:1.42. The total number of the neurones in the mamillary nucleus was 113500, and that of the anterior thalamic nucleus was 165600. The ratio of the neurones between these two nuclei was 1:1.46.According to the results mentioned above, the functional significance of the characteristic structure of the hippocampus was also discussed.