Investigations were carried out on 140 Wistar rats, five male in each group. The animals received intragastrically single doses of 1, 6 and 12 mg HgCl2/kg body weight. One hour after intoxication the rats were given in the same manner dose of Na125I/0.2 cm3, 95 kBq. I-125 activity of organs at 3, 6 h and 1, 2, 4, and 14 days after radioiodine administration indicated that body distribution of I-125 was significantly altered by the administration of mercury. The most dramatic changes were observed in the thyroid. These investigations showed that mercury may exert its toxic action through changes induced in the distribution of iodine in the organism.