Inhalation of radioactive metallic mercury vapor (203Hg0) in the mouse resulted in an accumulation of mercury in several organs where no specific uptake was observed after i.v. injection of inorganic mercury (203Hg2+). This was true for the whole respiratory epithelium (including the lung parenchyma), myocardium, brain, retina of the eye, adrenal cortex, corpora lutea of the ovary, epididymis, brown fat and thyroid gland. It is assumed that these organs have a high capacity for oxidizing Hg0 to Hg2+, which will then be retained in the tissues. Ethanol and aminotriazole (catalase inhibitors) decreased the concentration in several of these organs, although not in an exactly similar pattern. In the livers of non-treated animals, most of the inhaled mercury accumulated in the hepatocytes in the periphery of the lobuli (periportal region), close to where the blood vessels enter the liver parenchyma. Treatment with ethanol or aminotriazole increased the liver mercury content, with more or less all the hepatocytes apparently engaged in the oxidation of Hg0.