DOCA hypertension, renal hypertension, and cerebral hypertension were induced in Wistar rats. In the acute and chronic stages, noradrenaline storing cells of the adrenal medulla and cells of the zona fasciculata in the adrenal cortex were studied, both electron microscopically and histometrically, in order to compare them with their respective controls. 1. (a) In the acute stage of DOCA hypertension, (i) the number of noradrenaline storing granules was not different from that in the controls. Vesicles were about 1.6 times as numerous as those in the controls. Some of the granules and vesicles varied greatly in size. However, only a few of them probably representing initial secretory granules were noted. (ii) A large number of relatively large secretory granules of moderate electron density were found within the cytoplasm of cells of the zona fasciculata of the adrenal cortex. Both the number of the mitochondria and the number of ribosomes were decreased. (b) In the chronic stage of DOCA hypertension, (i) the nor-adrenaline storing granules and vesicles showed findings similar to those described in the acute stage. Especially noted were giant granules and vesicles. (ii) Cells of the zona fasciculata of the adrenal cortex also gave findings similar to those in the acute stage. 2. (a) In the acute stage of renal hypertension, (i) the number of noradrenaline storing granules was not significantly different from the controls. The number of vesicles was 1.7 times as great as that of the controls. Granules and vesicles varied in size and small ones were also found in abundance. (ii) In the cells of the zona fasciculata of the adrenal cortex, secretory granules probably representing lipid droplets were also found to be rather abundant as compared with controls. (b) In the chronic stage of renal hypertension, (i) no remarkable changes were noted in the number of noradrenaline storing granules and vesicles. Morphological changes were not remarkable except for slightly larger vesicles than those in the controls. (ii) In the cells of the zona fasciculata of the adrenal cortex, there were more abundant round secretory granules of moderate electron density than those in the controls. Dilatation and an increase of smooth-surfaced endoplasmic reticulum and ribosomes were noted. 3. (a) In the acute stage of cerebral hypertension, (i) the number of noradrenaline storing granules was unchanged and the number of vesicles was about 1.9 times that of the controls. As to the morphology, both granules and vesicles were larger than the controls. (ii) In the cells of the zona fasciculata of the adrenal cortex, large secretory granules of low electron density probably representing lipids were noted. Dilatation and an increase of smooth-surfaced endoplasmic reticulum and an increase of ribosomes were noted. (b) In the chronic stage of cerebral hypertension, (i) noradrenaline storing granules were increased (1.2 times those in the controls) and vesicles were also found abundantly (2.6 times those in the controls). Both granules and vesicles were variable in size and there were abundant small granules as well. (ii) In the cells of the zona fasciculata of the adrenal cortex, secretory granules of medium electron density probably representing lipids were found to be abundant. The smooth-surfaced endoplasmic reticula were di-