Norcantharidin (NCTD) is a small-molecule metastasis inhibitor without renal toxicity derived from a renal toxic compound cantharidin, which is found in blister beetles (Mylabris phalerata Pall.), commonly used in traditional Chinese medicine. The anti-metastatic capacity of NCTD is apparently through the downexpression of matrix metalloproteinase-9 (MMP-9) activity. The aim of this study was to clarify the transcriptional regulation of MMP-9 gene by NCTD in colorectal cancer CT-26 cells. NCTD not only downregulated MMP-9 mRNA and protein expression, but also inhibited gelatinase activity in a concentration- and time-dependent manner. In CT26 cells with transfection of cis-element reporter plasmids, NCTD treatment decreased reporter luciferase activity from a Sp1 construct, augmented with a NF-kappaB construct, but this did not occur with an AP-1 construct. Further transfecting with constructs containing wild-type or various mutant MMP-9 promoters in CT26 cells indicated that Sp1, but not the others, was required for NCTD-inhibition of MMP-9 promoter transactivation. More evidence by electrophoretic mobility shift assay demonstrated that NCTD inhibited the DNA-binding activity of Sp1. In addition, the increase effect of NF-kappaB-luciferase activity by NCTD may include the upexpression of nuclear STAT1 and result in competitive suppression of NF-kappaB-binding activity in MMP-9 promoter. In conclusion, the metastasis inhibitor NCTD downregulates MMP-9 expression by inhibiting Sp1 transcriptional activity in colorectal cancer CT26 cells.