Metallothionein (MT) is induced by various types of oxidative stress. However, whether or not MT is induced in renal ischemia/reperfusion injury, in which oxidative stress is believed to play a major role, remains unknown. The present study investigated MT expression in the kidneys of rats with ischmic acute renal failure (IARF). Rats were subjected to 60 min of bilateral renal ischemia followed by reperfusion. Renal MT mRNA expression was then analyzed by Northern blotting. MT expression in ischemic kidney was also localized by in situ hybridization and immunohistochemistry. Renal MT mRNA expression, which was barely detectable in the sham-operated control kidney, increased significantly at 3 h afer reperfsion, continued to increase to a maximal level at 24 h that was maintained for 48 h. The level of MT mRNA expression returned to that of the control by day 4. A morphological study revealed that MT was expressed exclusively in the renal tubular epithelial cells, which are the targets of ischemia/reperfusion injury, and that MT predominated in the outer medulla in the IARF rat kidney at transcriptional and translational levels. These results suggest that MT induced in the IARF rat kidney plays an important role in protecting renal cells against oxidative stress induced by ischemia/reperfusion.