We investigated the effects of testosterone and dihydrotestosterone on inflammatory response of iNOS and COX-2 expression in rat vascular smooth muscle cells. Rat vascular smooth muscle cells (VSMC) stimulated with bacterial lipopolysaccharide (LPS; 10μg/ml) for 24 hours were incubated with increasing amounts of testosterone and dihydrotestosterone (1 and 100 nM). LPS was found to induce inflammatory response of iNOS and COX-2 mRNA and protein in VSMC. These processes were affected by male sex steroid hormones. For 3 hours, however, pretreatment of the cells with 100 nM each of testosterone and dihydrotestosterone suppressed LPS induced iNOS and COX-2 protein expression. RT-PCR analysis revealed that testosterone and dihydrotestosterone did not inhibit mRNA expression of iNOS and COX-2 stimulated by 24 hours of LPS incubation. Proliferation rate was slower in VSMC treated with testosterone and dihydrotestosterone. Testosterone enhanced androgen receptor expression, and LPS significantly reduced androgen receptor protein expression in VSMC. These results indicate that the expression of both iNOS and COX-2 proteins was suppressed by testosterone and dihydrotestosterone in LPS stimulated VSMC and leading to reduction of vascular inflammation.