The author made cytological observations of the adrenal cortex in mice with leukemia or mammary cancer. The results of this experiment were as follows: 1. Adrenal glands of mice were, in general, hypotrophic and sudanophilic droplets which were also Schiff-positive or Schultze-positive increased markedly in number in zona fasciculata et glomerulosa. 2. In mice with mammary cancer, adrenal glands were hypertrophic and sudanophilic droplets which were also Schiff-positive or Schultze-positive decreased relatively in number in zona fasciculata et glomerulosa. 3. The foregoing evidence indicates that the adrenal cortex in leukemia mice is hypofunctional and in mammary cancer hyperfunctional.