Acute effects of methylmcrcuric chloride and mercuric chloride on thyroidal functions were examined. The organic mercurial concentration of 4×10^-5 M inhibited by 50% of Na⁺K⁺ ATPase in the membraneous preparation from the hog thyroid, and 6×10^-7 M of the inorganic mercurial showed the same extent of the inhibition. The Mg²⁺ATPase activity in the preparation was neither affected by CH₃HgCl up to a concentration of 2×10^-3 M, nor by HgCl₂ up to 1×10^-4 M. After an intraperitoneal injection to mice of 5 μg of mercurial per gram body weight daily for 2 consecutive days, the 4-hour and the 24-hour uptakes of ¹³¹I by the thyroids were partially reduced by both organic and inorganic mercurials. A significant reduction in percentages of labeled iodothyronines was demonstrated to suggest that mercurial may cause a coupling defect in the synthesis of iodothyronines. Incubation of hog thyroglobulin with 8×10^-3 M of methylmercuric chloride caused no observable aberration in slab disc electrophoreogram, but the protein was apparently denatured by the same concentration of mercuric chloride suggesting that thyroglobulin may carry a large binding capacity against either mercurial, but the inorganic mercurial can be more potent denaturant of the protein. The in vitro lysosomal hydrolysis of the mercurialpretreated rat thyroglobulin which was labeled with ¹²⁵I in vivo and fortified with the carrier hog thyroglobulin was not affected, but the direct addition of either mercurial in the medium resulted in a significant inhibition of the proteolytic action. Iodotyrosine deiodinase in the thyroid was inhibited by both mercurials in in vitro and in vivo systems. A partial reduction in the serum bound ¹³¹I-iodide in both mercurial treated groups was observed at 4 hours and 24 hours after the radioiodide administration. The blood thyroxine levels estimated by radioimmunoassay were quite reduced in the inorganic mercurial treated group and also moderately reduced in the methylmercurial treated group, indicating that the hormone secretion was affected by mercurials.