Influence of dietary protein levels on mercury (Hg) fate and on tissue metallothionein (MT) levels was investigated in mice. Twenty-four hr after single administration of mercuric chloride (2.5 mg Hg/kg, subcutaneous), the hepatic Hg concentration was enhanced by dietary protein deficiency, whereas the levels in other tissues and excrements were not affected. At that time, MT inductions by mercuric chloride in liver and kidney were suppressed by dietary protein deficiency, despite no observable differences in basal levels. Thus, Hg levels in the liver and kidney showed little correlation with MT levels. A further experiment demonstrated an enhancement of Hg concentration in the liver by dietary protein deficiency at 3 and 12 hr but not at 1 hr, and the Hg concentration in the kidney was transiently enhanced at 3 hr. Accordingly, the differences in Hg fate would arise considerably earlier, probably before MT induction. The present results suggest that dietary protein status modifies the fate of inorganic Hg, especially in the liver, probably independent of the differences in dietary protein level-dependent varying levels of MT.