Abstract: The effects of celastrol(Cel),isolated from Tripterygium wilfordii,on guinea pigsperm forward motility(FM),capacitation(Cap),the acrosome reaction(AR)and spermpenetration assay(SPA)were assessed in vitro.Cel(5 μg· ml^-1)was found to inhibit thesespermatozoal functions,and the inhibitions were proportional to the concentrations of Cel used.Thepotency of inhibition of Cel on the fertilizing ability in guinea pig spermatozoa in vitro seems to followthe order :Cap>FM>SPA>AR.The inhibitory effect appeared to be reversible after washing awayCel if the duration of exposure of spermatozoa to Cel was shorter than 3 h. In a comparative study,the inhibitory effects of Cel on guinea pig sperm FM and AR were significantly stronger than those ofgossypol acetic acid.