To examine the mechanisms involved in the progression of mercury chloride (HgCl2)-induced acute tubular necrosis (ATN), we investigated the histopathological changes and the expression of inducible nitric oxide synthase (iNOS) mRNA and protein in renal cortices of rats at 20 hours after exposure to HgCl2. The expression of iNOS mRNA was significantly augmented in renal cortices of rats with HgCl2-induced acute renal failure (ARF). Likewise, the induction of iNOS protein was observed in damaged proximal tubule epithelial cells of rats with HgCl2-induced ARF. Pretreatment of rats with iNOS inhibitor aminoguanidine, however, suppressed the development of proximal tubule epithelial cell injury and prevented an increase in blood urea nitrogen and serum creatinine as well as resulting in a marked fall in iNOS mRNA and protein in rats with HgCl2-induced ARF. These observations indicate that the induction of iNOS may play a role in the progression of HgCl2-induced ATN through the exacerbation of proximal tubule epithelial cell damage.