The greater part of the weight increase of the adrenal is due to growth of the cortex. However, the relative degree of medullary growth exceeds that of cortical growth. There is evidence of medullary hyperplasia, but the cortical growth may be due to cell hypertrophy alone.

In view of the known relationship between prolonged skeletal muscle work and the size of the adrenals, in which there is likewise a relatively greater growth of the medulla, the possibility that adrenal growth in experimental renal hypertension is a consequence of the metabolic demand of increased heart muscle work must be considered.

The hyperplastic, hypertrophic medulla of the renal hypertensive rat is presumed to be producing increased amounts of a hormone whose action is not known. The adrenal medulla is not necessary for the development of moderate renal hypertension though this does not exclude the possibility that it may be involved somehow in the maintainance of the hypertensive state.

Large, periodic-Schiff-positive, hyaline droplets are found in the cytoplasm of adrenal cortical cells with nuclear evidence of active function. There is some evidence that these indicate hypophyseal stimulation. Similar particles are found in the medulla, suggesting an interrelationship between the two parts of the adrenal.

The close resemblance of changes in adrenal weight, nuclear/cytoplasmic ratio, and nuclear and cytoplasmic structure, indicates that similar physiological and morphological controlling mechanisms are involved in the pathogenesis and maintainance of experimental renal hypertension and clinical essential hypertension.