Objective To explore the mechanism of ginsenoside Rg1 in alleviating heat stress-induced testicular injury in mice.Methods A total of 20 C57BL/6 male mice(6～8 weeks old)were randomly divided into 4 groups(n=5).The mice from the control group and heat stress(HS)group were intraperitoneally injected with 10 mL/(kg·d)0.9％normal saline for 14 d,while those in the HS+Rg1 group and the Rg1 group were given an intraperitoneal injection of 20 mg/(kg·d)for 14 d,and then on the 7th day after administration,the mice in the HS group and the HS+Rg1 group had the lower abdomen put into a 43 ℃ water bath for 30 min as a single heat stress after being anesthetized with 4％chloral hydrate.Mouse spermatocytes GC-2spd(ts)were divided into control group(routine culture for 48 h),HS group(placed in a 43 ℃ water bath for 30 min after 36 h of conventional culture,and cultured till the end of 48 h),HS+Rg1 group(50 μmol/L Rg1 treatment followed by heat stress injury),and Rg1 group(no heat stress injury).In 1 d after modeling,the eyeball blood samples were collected to detect serum testosterone with ELISA,and the testicles were extracted to observe the morphology and weighed to calculate the testicular index.HE staining was used to observe the histopathology of testis,and corresponding reagents and kits was employed to detect the content of malondialdehyde(MDA)and activities of catalase(CAT)and superoxide dismutase(SOD)in testis tissue.After the epididymal sperm were collected,the sperm concentration and motility were analyzed by computer-assisted sperm analysis(CASA)system.In in vitro experiments,cell apoptosis was detected with TUNEL staining,the protein levels of Nrf2,Keap1,HO-1,Bax,Bcl-2 and Caspase3 were detected with Western blotting,and the mRNA levels of GCLC,GCLM and NQO1 were detected by RT-qPCR.Results Rg1 prevented the decreases in testicular weight and testicular index caused by heat stress,reduced the damage of testicular tissue structure,prevented the decrease of sperm concentration and vitality,antagonized the decreasd number of Leydig cells and serum testosterone level,reduced the accumulation of MDA in testicular tissue,and enhanced the activities of CAT and SOD.Rg1 treatment alleviated the apoptosis of GC-2spd(ts)cells,down-regulated the expression of Bax,Caspase3 and Keap1 proteins,enhanced the expression of Bcl-2,Nrf2 and HO-1 proteins,and increased the transcriptional levels of Nrf2 target genes GCLC,GCLM and NQO1.Conclusion Rg1 has no significant effect on the structure and function of mouse testes,but it can effectively improve the ability of mouse testes to resist heat stress injury,which may be related to the activation of Nrf2 signaling pathway,the improvement of antioxidant enzyme activity,and the reduction of apoptosis of spermatogenic cells.