The mode of cell replacement in the adrenal cortex of the adult female mouse was studied bjr radioautography after labeling cells with tritiated thymidine. The fate of the labeled cells was followed in a series of animals killed at time periods up to twenty-one days after thymidine injection. It was found that the vast majority of labeled cells were located in the peripheral region of the cortex and that very few radioactive cells were present in the inner fasciculata and reticularis. Calculations made from cell counts indicated that the mitotically active outer cortex would be replaced within one to three months. Since no shift in the distribution pattern of the labeled cells occurred during the twenty-one day period studied it was concluded that no appreciable migration of cortical cells occurs. In accordance with these findings it was further concluded that the “escalator” concept should be abandoned.