1. Removal of one adrenal in the albino rat is followed by hypertrophy of the remaining gland. In the case of female rats 90 days of age this compensatory hypertrophy amounted to 61 per cent. 2. The increase in size of the whole gland is due entirely to hypertrophy of the cortex. This increase in cortical tissue amounted to approximately 91 per cent and was due in large part if not entirely to an increase in the size of the cells. 3. No increase in the size of the medulla or its cells was found.