A histochemical study was performed to determine the cellular localization of selenium in the adrenals from rats exposed to sodium selenite, as these organs were known to obtain the highest concentration in the rats. Male rats were treated either with 1-15 mg of sodium selenite for 14 days to 6 months in the drinking water, or with 1-20 mg as intraperitoneal injections. Selenium was shown to accumulate in both norepinephrenic and epinephrenic cells in a dose-dependent fashion. After oral exposure most deposits were found in the epinephrenic cells, whereas the norepinephrenic cells contained most in the injected animals. Fourteen days after a single injection (4 mg/kg), deposits could still be observed in the granules of the chromaffin cells. At the ultrastructural level, accumulations were primarily found in the chromaffin granules and to a lesser extent in the lysosomes. Selenium may possibly form bonds to endogenous zinc in the adrenals as has been suggested for the brain.