Based on epidemiological data, chronic exposure to high levels of inorganic arsenic in the drinking water is carcinogenic to the urinary bladder of humans. Recently, models have been developed involving transplacental administration of inorganic arsenic and subsequent administration of another substance that produces a low incidence of urogenital neoplasms. Administration of arsenite or arsenate in the diet or drinking water to fiveto eight-week-old mice or rats rapidly induces urothelial cytotoxicity and regenerative hyperplasia. In mice administered arsenite, we observed eosinophilic intracytoplasmic granules present in the urothelial cells. These granules were not present in urothelial cells of untreated mice or in treated or untreated rats. By transmission electron microscopy, the granules were located within the mitochondrial matrix, that is, mitochondrial inclusions. Arsenic, primarily as arsenite, was present in partially purified mitochondria containing these granules. Cells containing the granules were not usually associated with degenerative changes. Lack of these granules in rats suggests that they are not necessary for inorganic arsenic-induced urothelial cytotoxicity or hyperplasia. These granules have also been observed with exposures to other metals in other tissues and other species, suggesting that they represent a protective mechanism against metal-induced toxicity.Keywords: mitochondrial granules; urinary bladder; arsenite; cytotoxicity; hyperplasia diethylstilbestrol or tamoxifen, some eventually develop urogenital neoplasms. Furthermore, Simeonova et al. (2000) demonstrated that As III administered in the drinking water (0.01%) to C57BL x BDA2 mice for four weeks produced evidence of urinary bladder urothelial simple hyperplasia.In a previous short-term study involving administration of arsenate (As V ) in the drinking water to rats, we also observed urothelial hyperplasia (Lu et al. 2007). In subsequent experiments involving administration of As III and As V to mice, we noticed the presence of intracytoplasmic eosinophilic granules in the mouse urothelium, especially in the superficial cell layer (Suzuki et al. in press). In the present study we examined this phenomenon in greater detail in urothelial cells of mice administered As III . By transmission electron microscopy (TEM), we demonstrated that the granules are within the mitochondria. The urothelium in these mice also showed evidence of hyperplasia. Interestingly, in contrast to mice, treatment of rats with 200 mg/L sodium arsenite (NaAs III ) in the food did not produce mitochondrial granules (MG) despite detection of simple hyperplasia in the urinary bladder epithelium. MG have been observed in various tissues in different species subjected to exposure to certain heavy metal cations. The granules are mainly associated with the mitochondrial inner membrane and contain the metal within the matrix of the mitochondria (Brown et al. 1985;Jacob et al. 1994). The mitochondrion is an INTRODUCTION