AbstractThe fine structure of adrenal cortices of opossums maintained on a sodium deficient diet for 21 days has been examined. After this treatment, the thickness of the zona glomerulosa is increased due to hypertrophy and hyperplasia of the parenchymal cells. After 21 days on the low sodium diet, the glomerulosa cells are characterized by large accumulations of smooth surfaced endoplasmic reticulum. This appearance is in marked contrast to the sparse representation of smooth surfaced reticulum in control animals. The rough surfaced endoplasmic reticulum becomes organized into long flattened cisternal profiles in parallel array and appears to be increased in quantity. The Golgi complex is also hypertrophied but marked changes in other organelles were not observed. Cells of the zona fasciculata appear to be unaffected by sodium depletion. Large lipid droplets accumulate in the zona reticularis of experimental animals but this zone is otherwise comparable to control animals. The bearing of these results on hypotheses of physiological control of secretion of the zona glomerulosa are discussed.