Mifepristone (RU486) ,a synthetic steroid compound that induces abortion,was recently found to potentially pass into human milk.This study aimed to determine the effects of mifepristone administration to lactating mice on the development and reproduction of their progeny.Lactating mice (n= 8 per group) were gavage fed with mifepristone (8 mg/kg) daily for 4 days either from days 1-4 or days 7-10 of lactation,controls received only peanut oil.Growth,mortality rate,organ weight to body weight ratio and sex hormones at 20,40 and 60 days,fertility of these F1 progeny and litter size,sex ratio,and mortality rate of the second generation were recorded.No significant differences were observed in the average body weight,mortality rate of pups,testis coefficients in male pups or uterus and ovaries of F1 progeny in adulthood,and subsequent fertility of both male and female pups in comparison with the control.Testosterone concentration was significantly different between treated groups and the control at day 60.Moreover,the reproductive capacity of the F1 pups was unaffected by 4 days exposure to mifepristone via their mothers milk-time to birth of F2 pups,litter size,sex ratio,and mortality rate were similar to control F1 pups.This study has shown that treating lactating mice with 8 mg/kg mifepristone did not affect the development and fertility of female and male pups.These results suggest that women who undergo medical abortion using mifepristone at routine doses may safely continue to breastfeed their infants.