The effect of intestinal flora on the absorption and dispositon of mercury in tissues was investigated using conventional rats, and rats treated with antibiotics to eliminate their gut flora. Antibiotic-treated rats given [203Hg]-labeled methylmercuric chloride orally had significantly more mercury in their tissues, especially in kidney, brain, lung, blood, and skeletal muscle, and also excreted less mercury in the feces than conventional rats. Furthermore, in the kidneys of the antibiotic-treated rats, the proportion of mercury present as organic mercury was greater than in the kidneys of the conventional rats. The results suppport the hypothesis that the metabolism of methylmercuric chloride by the gut flora reduces the tissue content of mercury. When rats were administered 10 mg methylmercuric chloride/kg . day for 6 days, four of five of those given antibiotics developed neurological symptoms of toxicity, whereas only one of five conventional rats given methylmercuric chloride was affected.