Few studies have examined the effects of isoflavones and particularly equol, a metabolite of the isoflavone daidzein, on bone formation during the growth period in animals. The present study investigated the effects of orally administered daidzein or equol on bone formation and bone mineral density in growing female rats. Female Sprague-Dawley rats, aged 3 weeks, were divided into four groups (n = 8 per group) as follows: rats were orally administered a corn oil, 8 mg/day of daidzein, 4 mg/day of equol or 8 mg/day of equol in corn oil for 4 weeks. Daidzein and equol increased the bone mineral density of growing female rats by stimulating bone formation without exhibiting a substantial effect on the weight of their reproductive organs. Bone growth caused by increased bone mineralizing surface and bone formation rate in rats administered with equol was approximately twice that of rats administered with daidzein. These results suggest that equol might be more efficient than daidzein for bone formation in growing female rats. Copyright © 2015 John Wiley & Sons, Ltd.