Sodium butyrate, a histone deacetylase inhibitor, has been shown to induce differentiation of many cancer cells and senescence-like state of human fibroblasts. Here we show that sodium butyrate also induces senescence-like state of NIH3T3 cells. The treated cells were blocked at G1 phase and featured morphologically like senescent cells with enlarged cytoplasm and multiple nuclei. The expression of p21(WAF/CIP1) (p21) increased after sodium butyrate treatment at transcriptional level. To analyze the induction of promoter activity, we isolated 4.6 kb of murine p21 promoter and inserted it upstream of a luciferase reporter gene. When this construct was transiently transfected into NIH3T3 cells, sodium butyrate enhanced the luciferase activity. p53 independency of sodium butyrate-inducible p21 promoter activity was confirmed by using the deletion mutants lacking p53 binding sites and p53 deficient cells in transfection experiments.