There is a general agreement that isoflavones can be beneficial to health in adults. However, isoflavones are well known as endocrine-disrupting chemicals. It should be considered that soy foods might adversely affect the reproductive system and infants. The aim of this study was to evaluate the effects of genistein, an isoflavone, on dams and their offspring. Maternal rats were fed diets containing genistein at levels of 0 and 0.5 g/kg diet from pregnancy day 5 to postnatal day 13. No effects of genistein on the delivery, anogenital distance, reproductive organ weight, and body weight of the infants at birth were observed. There were no consistent effects on suckling pups after continuous genistein exposure during their fetal and suckling stages through their mothers, and there was no difference in effects according to the periods of exposure during pregnancy and lactation. We also observed no significant effect on the growth of offspring after weaning. Moreover, while we observed that the serum concentration of triiodothyronine (T3) in dams decreased, the result was a tendency, not a significant decrease. Our study suggested that maternal ingestion of genistein might have not induced serious adverse effects on dams, fetuses, infants or offspring during growth. However, the results indicated in many papers suggest the necessity of further study on the safety of genistein.