Histone deacetylase inhibitor-induced expression of p21WAF1 is p53 independent. In the present study, we provide evidence that trichostatin A (TSA), a specific inhibitor of histone deacetylase, can elevate H3 and H4 acetylation and p21WAF1 expression in NIH3T3 cells at first. To identify the transcription factor which is responsible for histone deacetylase inhibitor-induced expression of p21WAF1 and understand the potential events occurred during this process, we analyze the response of the mouse p21WAF1 promoter to TSA in detail. The region responsive to TSA treatment in the p21 promoter is located -100 bp upstream from transcription initiation site and contains a GC-box. The mutation introduced into this GC-box decreases most of the basal and TSA-induced promoter activity. The results from gel-shift assay show that Sp1 and Sp3 bind to this GC-rich region. Cotransfection with Sp1 and/or Sp3 expression constructs elevate both basal and induced promoter activity, and this elevation is dependent on the present of the GC-box. By contrast, cotransfection with reverse oriented Sp1 or Sp3 cDNA decreased basal and induced-promoter activity, as well as GC-box dependency. These findings provide physical and functional evidence which strongly indicated that both Sp1 and Sp3 are responsible for TSA-induced transactivation of the murine p21WAF1 promoter in NIH3T3 cells.