BACKGROUND: Vascular smooth muscle cell (VSMC) proliferation is a major pathologic finding of atherosclerotic vessels in diabetes mellitus. Lipopolysaccharide (LPS) inhibits the VSMC proliferation by NO production via iNOS expression. This study attempted to investigate the effect of LPS on the glucose-induced proliferation of VSMC and its mechanism of action. The effects of insulin on glucose induced VSMC proliferation and on the expression of iNOS were also investigated. METHODS: VSMCs were primarily cultured from rat aorta. A proliferation assay for VSMC was performed by a cell count. The concentrations of nitrite in culture media were measured using the Griess reaction. Western blots were performed to analyze for iNOS protein. RESULTS: D-glucose induced VSMC proliferation in a concentration-dependent manner. LPS inhibited the D-glucose induced VSMC proliferation by increasing ing nitrite production. Insulin suppressed the D-glucose induced VSMC proliferation and potentiated the LPS-induced inhibition of VSMC proliferation by increasing the nitrite production. Insulin potentiated the LPS-induced expression of iNOS. CONCLUSION: These results suggest that in diabetes mellitus, glucose induces VSMC proliferation, and LPS and insulin inhibit the stimulatory action of glucose on VSMC proliferation, and insulin potentiates the inhibitory action of LPS on VSMC proliferation via a increase in the expression of iNOS.