Atherogenesis is a chronic inflammatory response and intercellular adhesion molecule (ICAM-1) induced by cytokines plays a role in this event. In this study, the molecular mechanisms of tumor neurosis factor alpha (TNFalpha)- and IL-6-induced ICAM-1 gene expression in endothelial cells (ECs) were examined. ECs infected with adenovirus carrying the dominant negative mutant of Rac (Ad-RacN17) exhibited inhibition in both TNFalpha- and IL-6-induced ICAM-1 expression. Consistently, ECs transfected with RacN17 inhibited both TNFalpha- and IL-6-induced ICAM-1 promoter activities. Functional analysis of ICAM-1 promoter, however, indicated that the cis-acting elements in response to TNFalpha and IL-6 are different. The NFkappaB binding site in the ICAM-1 promoter region was crucial for TNFalpha-induced ICAM-1 expression but not for the induction by IL-6. ECs infected with Ad-RacN17 attenuated the TNFalpha-induced NFkappaB binding activity. In contrast, IL-6 activated a transcriptional factor, signal transducer and activator of transcription-3 (Stat3) via the phosphorylation of Tyr705 at Stat3. ECs transfected with the dominant negative mutant of Stat3 (Stat3F) demonstrated that Stat3 was required for IL-6-induced ICAM-1 gene expression. Interestingly, the phosphorylation of Tyr705 and Ser727 in Stat3 was greatly inhibited in IL-6-treated ECs previously infected with Ad-RacN17. Our data strongly indicated that ICAM-1 gene induction by TNFalpha and IL-6 is mediated mainly via NFkappaB and Stat3, respectively and Rac1 appears to play a central role in modulating cytokine-induced ICAM-1 expression in ECs.