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Body weight and blood pressure were measured in the normotensive Wistar rats with the systolic blood pressure below 149mm.Hg and spontaneously hypertensive rats by Okamoto and Aoki. Animals, from both groups, were killed at 40-50 days (prehypertensive stage), 4-5 months (after a moderate duration of hypertension), and 12-14 months of ages (after a long duration of hypertension). Endocrine organs were weighed and studied histometrically and histopathologically.The results in the hypertensive animals were as follows.(1) Some increases in weight of the pituitary and in the percentage of basophils in the anterior lobe were noted at the prehypertensive stage. These changes were exaggerated as hypertension persisted. The hypertrophy and the change similar to hyaline change were observed in basophils.(2) In the adrenals, the area of the glomerular zone and the summed area of the fascicular and reticular zones were increased. The hypertrophy and increase in lipid content of the glomerular and fascicular zones were detected even at the prehypertensi.ve stage. With persistence of hypertension these changes became more marked, and increase in weight of adrenals, irregularity of cell cords of the fascicular zone, decrease in width of the reticular zone were demonstrated. The increase in lipid content was remarkable in the glomerular zone and moderate in the fascicular zone but more remarkable in groups which were complicated by periarteritis nodosa or nephrosclerosis.(3) In the thyroid, the increase in area of follicle cells, follicle and lumen, and irregularity in shape of follicles were found at the prehypertensive stage. As hypertension persisted these changes became more remarkable. Weight of the thyroid increased and the colloid content decreased. The histological changes were compatible with that of struma parenchymatosa macrofollicularis.(4) In the pancreas and gonads, there was no significant difference between the hypertensive and normotensive groups.(5) The present study has demonstrated that the pituitary anterior lobe, adrenal cortex and thyroid have significant roles in the development and maintenance of hypertension in the spontaneously hypertensive rats.