With the materials of healthy adult rats, mice, rabbits, guinea pigs, golden hamsters, dogs, cats, oxen, pigs, horses and goats, cytological observations were made to examine the interrelationship of Zona glomerulosa to Zona fasciculata. The adrenal glands were fixed with LEVI's solution and serial sections stained with HEIDENHAIN's iron-hematoxylin pretreated with RUBASCHKIN's bleaching procedure. The results obtained were as follows.The typical glomeruli consisting of numerous cells with obsecure contour occupy Zona glomerulosa of rats. The basement membranes enclose the glomeruli to make them independent; the intermediary or transitional zone which is regarded as the particular layer composed of a few ranges of dark shrunken cells is situated between Zona glomerulosa and Zona fasciculata. This zone is not a comprehensive closure membrane, but has many defects through which the cell cords ascend perpendicularly to the capsule with the consequent combination of Zona glomerulosa with Zona fasciculata. At the uppermost area of Zona glomerulosa are present small glomeruli which consist of a few cells appearing to be too infantile to synthesize fat granules, whereas a mojority of cells were loaded with fat granules both at the deepest area of Zona glomerulosa and at the upper part of Zona fasciculata (designated as Zona spongiosa). In comparision to the other animals which will be described below, it has been learned that the transitional zone would be merely inherent to the rat, despite it is not a complete sudanophobic zone, but contains constantly much or less osmiophilic fat granules in objection to the appointment of previous investigators.In Zona glomerulosa of mice there were observed the reduction in number of cross-sections of the glomeruli. This would be referable to the fact that the cell cords of Zona fasciculata go upwards to the capsule and reflect down, presenting the fasciculo-arcuata pattern. Therefore these arrangements can precisely account for the absence of particular transitional zone. The cell cords and their internal structures correspond approximately in many respects to those in rats.Zona glomerulosa of rabbits is no longer worthy of its designation, since the cell cords protrude to the capsule and turn in forming a dome conditioned on the development of Zona fasciculo-arcuata. It was our common observation that the adrenal cortex of rabbits is lacked with the transitional zone. Under detail examination, however, we could enumerate two special layers shadowed with granular elements: The one is an immature cell layer connected to the capsule, in whose cell basement were deposited abundant minute granules and mitochondria, and the other is a dense cell layer consisting of a mass of nuclei whose cytoplasm is also charged with the faintly stained granules. Two layers were susceptible to be distributed transversally in parallel to the capsule. The rabbit is characterized by the numerous fat granules and by simultaneous deficiency of stainable ones in all cortical areas but those in above layers.The cell cords of Zona fasciculata in guinea pigs protrude irregularly upwards and terminate into the wide capsule at various places as the blind tips. The thickened basement membranes surrounding the cords are presumably originated from the ramifications of interstitial tissues of the capsule. The transitional zone does not take places anywhere in those cords. It can be considered to be the features provided with the guinea pig that the cortical cells contain abundant stainable granules instead of fat droplets with the exception of occassional occurrence of huge fat droplets. This is followed by the acquirement of their dark tone of cytoplasm.In golden hamsters it was apparent that Zona glomerulosa is involved in a mass of projections of the cell cords of Zona fasciculata. Beneath the capsule the cord forms a dome responsible for the development of Zona arcuata.