To clarify the functions of the renal metallothioneins (MTs) in rats following ischemic acute renal failure, the expression of MT mRNA and the localization of MT in rats undergoing 60 min of bilateral renal artery clamp ischemia followed by reperfusion was examined by Northern blot analysis, in situ hybridization and immunohistochemical analysis. Induction of maximum at 1 to 3 days followed by a gradual decrease. Histlogically, both MT mRNA and MT were detected in renal tubular cells intensely and specifically in the outer medulla at 1 day after release of the clamp. These findings suggest that MTs protect the tubular cells by scavenging the free radicals, which probably cause renal tubular cell damage by ischemia-reperfusion, and that MTs provide the zinc ions to zinc-requiring apo-enzymes necessary for healing and regenerating renal tubular cells.