The goal of this study was to clarify the role of p21, a cyclin-dependent kinase inhibitor, in acute renal failure (ARF). This was accomplished with the examination of the renal expression of p21 in cisplatin (CDDP)-induced ARF and in rechallenge injury with CDDP. The injection of CDDP (5 mg/kg) into rats induced increases in serum creatinine and tubular damage and the number of in situ DNA nick end labeling-positive cells, which peaked at day 5, followed by recovery to control levels by day 14. The rechallenge with the same dose of CDDP 14 d after the first dose of CDDP induced significantly less injury and no significant increase in in situ DNA nick end labeling-positive cells. The first CDDP dose significantly increased p53-positive nuclei at day 1, which disappeared by day 5, and the number of p21-positive nuclei, which had two peaks on days 3 and 9. The number of proliferating cell nuclear antigen (PCNA)-positive nuclei peaked at days 3 and 12. A significant increase in the incorporation of 5-bromo 2'-deoxyuridine (BrdU) was found at day 5 and peaked at day 7. The second injection of CDDP induced significant increases in the number of p21-, p53-, and PCNA-positive nuclei within 2 d but did not affect the incorporation of BRDU: These findings suggested that (1) CDDP induced two peaks of the increase in p21; (2) the first peak occurred shortly after CDDP and was accompanied by overexpression of p53 and PCNA but not with BrdU incorporation, possibly reflecting G1 arrest and DNA repair; (3) the second peak of p21 occurred through an p53-independent pathway and may contribute to cell differentiation; and (4) the overexpression of p21 and PCNA in rechallenge injury may contribute to acquired resistance in CDDP-induced ARF via enhanced DNA repair.