Intoxication effect on mouse thyroid by prolonged administration of either CH3HgCl or HgCl2 was studied. It was found by giving CH3 203HgCl and 203HgCl2 through stomach intubation at either a single or a 30 d treatment that thyroid is a moderately susceptible organ to both mercurials. Animals were given 50, 100 and 150 micrograms/d of either mercurial in drinking water for a month. At the lowest amount of HgCl2, body weight was increased, whereas at the highest dose, there was a transient delay in growth. With lower amounts of CH3HgCl, no change in growth was observed. However, at the highest amount, a severe growth inhibition occurred. The thyroid weight was unaffected by lower amounts, but was significantly reduced by 150 micrograms/d of either mercurial. The 24 h radioiodide uptake in the thyroid, expressed by cpm/mg organ weight, was reduced by lower levels of mercurials. CH3HgCl and HgCl2 suppressed the rate of radioiodide incorporation into the iodothyronine fraction, but not into the iodotyrosine fractions, indicating that mercurials do not interfere with organification of iodide but do inhibit the coupling process. Serum thyroxine (T4) level was affected by mercurials, but serum triiodothyronine (T3) was not. This result suggested that even thyroidal secretion of T4 was inhibited by mercurials, but the peripheral conversion of T4 to T3 may not be affected in the maintenance of an active hormone level.