Objectives : Baicalein (3,3', 4', 5, 6-pentahydroxyflavone), a type of flavonoid, is a well-known antioxidant and anti-inflammatory ingredient found in Scutellaria baicalensis root. The aim of this study is to investigate the effect of baicalein on nitric oxide (NO) production in TM3 mouse Leydig cells stimulated by indomethacin (IN). Methods : TM3 cells were treated with IN (0.5 μM) and baicalein at concentrations of 12.5, 25, 50, and 100 μM for 24 hr, 40 hr, 42 hr, 44 hr, and 64 hr. After treatments, cell viabilities were measured with the modified MTT assay. The production of nitric oxide in cells was measured by Griess reagent assay. Results : Baicalein showed no cytotoxicity on IN-stimulated TM3. NO production in IN-stimulated TM3 treated for 24 hr with baicalein at concentrations of 12.5, 25, 50, and 100 μM was 95.8%, 94.86%, 89.97%, and 81.52% of the control group treated with IN only, respectively; NO production for 40 hr was 97.34%, 97.34%, 95.15%, and 87.42%, respectively; NO production for 42 hr was 89.12%, 90.14%, 89.74%, and 90.26%, respectively; NO production for 44 hr was 83.83%, 84.94%, 85.65%, and 86.85%, respectively; NO production for 64 hr was 94.12%, 95.38%, 94.21%, and 94.12%, respectively. Specifically, baicalein at concentrations of 12.5, 25, and 50 have been shown to most efficiently inhibit NO productions in 48 hr of treatment. Conclusions : Baicalein might have anti-toxicant effect on Leydig cells related with its inhibition of NO production in Leydig cells stimulated with IN.