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This study evaluated the effects of mifepristone and quinestrol on the fertility of female Brandts voles (Lasiopodomys brandtii) .Either mifepristone (4mg/kg) or quinestrol (4 mg/kg) was administered in 0.8ml peanut oil by gavage for 3 days during pre-mating,early pregnancy (5 days since mating) ,or late pregnancy (15 days since mating) to 6 groups of 8 animals.A control group received peanut oil only.All females were mated with fertile males at 5 days and again at 12 days if required.Females treated with mifepristone during early or late pregnancy did not breed,while the fertility of the pre-mating treated group was not different to the controls in terms of reproductive rate,litter size,average body mass at birth,and survival rate.Quinestrol treatment during pre-mating and early or late pregnancy completely inhibited fertility.During the second mating period,the reproductive rates were 44.4% (early pregnancy quinestrol treatment) ,25% (late pregnancy quinestrol treatment) ,66.7% (early pregnancy mifepristone treatment) ,and 50% (late pregnancy mifepristone treatment) compared to the control group.No significant differences were observed in reproductive rate,ovarian or uterine weights,and progesterone or estradiol concentrations in the 4 groups when tissues were collected at 20 days after second mating.The results suggest that mifepristone and quinestrol treatment during early or late pregnancy had similar inhibitory effects on reproduction.During the pre-mating period quinestrol treatment showed a greater inhibitory effect than mifepristone treatment,which suggests that the reproductive processes of Brandts voles were more sensitive to quinestrol than to mifepristone.