Cyclosporin A (CsA) is an immunosuppressant with severe side effects including gingival overgrowth. We have previously reported that CsA impairs the activity of the lysosomal enzymes cathepsin B and L in human gingival fibroblasts (HGFs). Here, we have examined the effects of CsA on the DNA-binding activity of the cyclic AMP response element-binding protein (CREB) and cell viability, and the effects of CREB on cathepsin B and L synthesis and activity in HGFs. We have confirmed that CsA down-regulates cathepsin B and L synthesis. Further, CsA has no effect on cell viability and dramatically impairs CREB-DNA binding activity. Importantly, the synthesis of cathepsin B and L is down-regulated, and their activity is also significantly impaired in HGFs transfected with plasmid expressing dominant-negative CREB. These results suggest that CREB is essential for the CsA-mediated down-regulation of cathepsin B and L synthesis in HGFs.