Accumulations of mercury have been demonstrated in adrenal glands by light and electron microscopy with a highly sensitive histochemical technique. Rats were exposed to methyl mercury in drinking water (20 mg/l) for 7-180 days, or were given intraperitoneal injections of methyl mercury (daily dose 100 or 200 micrograms). The amount and location of the mercury deposits were dependent upon the exposure time, the method of administration and the amount administered. In rats exposed to methyl mercury in drinking water, accumulations were often observed in both the zona glomerulosa and reticularis. They appeared first in the zona glomerulosa of animals treated for 1 week. In the zona fasciculata, deposits were observed only in the animals treated for 50 to 180 days. In animals treated for 180 days the cytoplasm of the cells in the zona fasciculata was heavily vacuolated and distinct necrotic cells were observed in other cortical zones. In the chromaffin cells, a slight increase in the amount of deposits was observed with increasing exposure time. Both epinephrenic and norepinephrenic cells contained deposits. Only a few deposits were observed in the cortical and chromaffin cells of animals treated with intraperitoneal injections. Ultrastructural deposits were observed in the lysosomes of cortical cells and in both lysosomes and secretory granules of chromaffin cells.