Isoflavones are phytoestrogens which have estrogen-like activity in animals and humans with structure similar to 17-β-estradiol and capable of binding to estrogen receptors (ERs).This experiment was conducted to demonstrate the effect of isoflavones on estradiol concentration in blood and its influence on the productive performances in ISA Brown laying hens fed with supplemented feed with large amount of isoflavones. Isoflavones as non steroid phytoestrogens were added in large amounts to the feed of laying hens in 0, 1000, 2000 and 3000 mg kg-1 of feed. Estradiol concentration in blood of the isoflavone-treated chickens was significantly higher in comparison with the control group (P<0.01). The supplemented isoflavones in the feed have significant improvement on the egg weight during the 1st month (P<0.05) and 2nd month (P<0.01) and also for the whole experimental period (P<0.05). Further studies are required to investigate the effect of isoflavones on other reproductive hormones (follicle stimulating hormone (FSH) and progesterone) which are related with the egg production. Manuscript profile