Objective:To study regulation mechanism of GnRH agonist(GnRHa) on testosterone synthesis in rat Leydig cells. Methods:The primary rat Leydig cells were cultured 24 h in vitro, and then treated with GnRHa (10-7 mol/L) at varied time courses (6,12,24,36,48 h);the expression of 3β-HSD protein was analyzed by Western blot. After stimulation with GnRHa and PD98059 (50 μmol/L),the specific inhibitor of ERK1/2,in Leydig cells,the expression of 3β-HSD and the level of testosterone were also investigated. Results:Treatment with GnRHa resulted in 3β-HSD expression in a time-dependent manner. 3β-HSD expression was significantly increased 45% over control after 12 h of treatment with GnRHa (P < 0.05),maximally elevated 1.0-fold over control at 24 h (P < 0.05),and then returned to basic level after 48 h. We also found the activation of 3β-HSD induced by GnRHa was markedly reversed in the presence of PD98059 (61% compared with GnRHa group,P < 0.05),following the significant decrease of testosterone (45% compared with GnRHa group,P < 0.05). Conclusion:The results suggest that GnRHa regulates the expression of 3β-HSD through ERK1/2,and further induces testosterone synthesis.