Members of the Rho family of small GTPases have been recently implicated in inflammatory signaling. We examined the effect of in vivo inhibition of Rho kinase on atherogenesis in mice. Low-density lipoprotein receptor (LDLR) knockout (KO) mice fed a cholate-free high-fat diet received daily intraperitoneal injection of saline (n=8, control group) or Y-27632 (30 mg/kg, n=9), a specific Rho kinase inhibitor. After 9 weeks, Y-27632 treatment resulted in significant in vivo inhibition of Rho kinase activity (P=0.004). Body weights, arterial blood pressures, and plasma cholesterol levels were comparable in both groups. Atherosclerotic lesion size in the aortic sinus and thoracic aorta of mice treated with Y-27632 was reduced by respectively 35% and 29% in comparison with the saline-treated animals (P=0.006 and P=0.03, respectively). This was associated with a significant reduction in T lymphocyte accumulation (P=0.035) and expression of p65 subunit of NF-kappaB within plaques (P<0.05). In vitro, treatment with Y-27632 inhibited p65 phosphorylation and degradation of IkappaBalpha in mouse peritoneal macrophages and significantly inhibited concanavalin A-induced proliferation of spleen-derived T cells (P<0.001). In conclusion, inhibition of Rho kinase significantly limits early atherosclerotic plaque development in the LDLR KO mice. This study identifies Rho kinase inhibitors as potential candidates for the treatment of atherosclerosis.