We investigated the involvement of the RhoA/Rho-associated kinase (ROCK) pathway in regulating ICAM-1 expression in endothelial cells by the procoagulant, thrombin. Exposure of HUVECs to C3 exoenzyme, a selective inhibitor of Rho, markedly reduced thrombin-induced ICAM-1 expression. Inhibition of ROCK, the downstream effector of Rho, also prevented thrombin-induced ICAM-1 expression. Blockade of thrombin-induced ICAM-1 expression was secondary to inhibition of NF-κB activity, the key regulator of ICAM-1 expression in endothelial cells. In parallel studies we observed that inhibition of the RhoA/ROCK pathway by the same pharmacological and genetic approaches failed to inhibit TNF-α-induced NF-κB activation and ICAM-1 expression. The effect of RhoA/ROCK inhibition on thrombin-induced NF-κB activation was secondary to inhibition of IκB kinase activation and subsequent IκBα degradation and nuclear uptake and the DNA binding of NF-κB. Inhibition of the RhoA/ROCK pathway also prevented phosphorylation of Ser 536 within the transactivation domain 1 of NF-κB p65/RelA, a critical event conferring transcriptional competency to the bound NF-κB. Thus, the RhoA/ROCK pathway signals thrombin-induced ICAM-1 expression through the activation of IκB kinase, which promotes NF-κB binding to ICAM-1 promoter and phosphorylation of RelA/p65, thus mediating the transcriptional activation of bound NF-κB.