Cimicifuga heracleifolia extract (CHE) was investigated for its effects on the release of nitric oxide (NO) and at the level of inducible nitric oxide synthase (iNOS) gene expression in mouse macrophages. We found that C. heracleifolia elicited a dose-dependent increase in NO production and the level of iNOS mRNA. Since, iNOS transcription has been shown to be under the control of the transcription factor NF-kB, the effects of CHE on NF-kB activation were examined. Transient expression assays with NF-kB binding sites linked to the luciferase gene revealed that the increased level of iNOS mRNA, induced by CHE, was mediated by the NF-kB transcription factor complex. By using DNA fragments containing the NF-kB binding sequence, CHE was shown to activate the protein/DNA binding of NF-kB to its cognate site, as measured by electrophoretic mobility shift assay. These results demonstrate that C. heracleifolia stimulates NO production and is able to up-regulate iNOS expression through NF-kB transactivation.