The purpose of this study was to evaluate whether upregulated p21, cell cycle-inhibitory protein, contributes to cisplatin (CDDP)-induced acute renal failure (ARF) and to acquired resistance to rechallenge injury with CDDP in rats. ARF was induced in rats by injection of CDDP (5 mg/kg) and rechallenge injury to CDDP by the same dose of CDDP 14 days after the first CDDP injection. Rats were treated with p21 antisense oligodeoxynucleotide (ODN) or its vehicle, p21 sense ODN, every 36 hours from days 0 to 5 for single CDDP and from days 13 to 19 for rechallenge injury, and sacrificed at days 3, 5, 16 or 19. The uptake of FITC-labeled p21 antisense ODNs by cortical proximal tubule (PT) cells was much more than by PT cells in the outer stripe of outer medulla (OSOM). Administration of antisense induced partial downregulation of p21 mRNA and protein levels in the whole kidneys of CDDP single treatment and its rechallenge injury. Antisense significantly aggravated PT necrosis and decreased the number of p21-positive PT cells in the cortex but not in the OSOM both in CDDP-induced ARF and its rechallenge injury. However, antisense did not alter serum creatinine (Scr) and blood urea nitrogen (BUN) level. Our findings suggested that p21 plays, at least in part, a cytoprotective role in cortical PTs exposed to CDDP though this does not contribute to renal dysfunction when judged by Scr and BUN level. Since antisense may not adequately be taken up and/or function in PTs in the OSOM, the role of p21 in PTs in the OSOM in CDDP-induced ARF remains to be clarified.