There is considerable concern that isoflavones,such as genistein in fish feed composed of soybean protein,affects somatic growth in fish.Our previous works demonstrated that 30 and 300 μg/g dietary genistein had no significant effect on growth performance in Nile tilapia(Oreochromis niloticus),but the higher level of genistein(3 000 μg/g) significantly depressed growth.This study was conducted to further examine the effects of dietary genistein on the endocrine disruption on growth hormone/insulin-like growth factor-I(GH/IGF-I) axis in Nile tilapia(O.niloticus).Juvenile fish were fed by hand twice daily to satiation with one of four isonitrogenous and isoenergetic diets,each containing either 0,30,300 or3 000 μg/g genistein.Following an 8-week feeding period,plasma GH and IGF-I levels were investigated by radioimmunoassay and gene expression levels of gh,ghrelin,gnrhs,ghr,npy,npyrs,pacap,ghrs,igf-I,igf-Ir,and igfbp3 were examined by real-time PCR.The results show that no significant change in plasma GH and IGF-I levels in fish fed with diets containing 30 μg/gand 300 μg/g genistein.mRNA expression of genes along the GH/IGF-I axis remained unaffected,except for igf-Ir,which was stimulated by the 300 μg/g genistein diet.While in fish fed the 3 000 μg/g genistein diet,the plasma GH and IGF-I levels decreased,and mRNA expression of gh,ghr2,npyr1,igf-I,and igf-Ir were also significantly depressed.In contrast,npy and igfbp3 mRNA expression were enhanced.This study provides convincing evidence for growth impediment by genistein by disturbing the GH/IGF-I axis in Nile tilapia O.niloticus. There is considerable concern that isoflavones, such as genistein in fish feed composed of soybean protein, as somatic growth in fish. Our previous work of that 30 and 300 μg / g dietary genistein had no significant effect on growth performance in Nile tilapia (Oreochromis niloticus ), but the higher level of genistein (3 000 μg / g) significantly depressed growth. This study was conducted to further examine the effects of dietary genistein on the endocrine disruption on growth hormone / insulin-like growth factor-I (GH / IGF -I) axis in Nile tilapia (O. niloticus) .Juvenile fish were fed by hand twice daily to satiation with one of four isonitrogenous and isoenergetic diets, each containing either 0, 30, 300 or 3 000 μg / g genistein.Following an 8-week feeding period, plasma GH and IGF-I levels were investigated by radioimmunoassay and gene expression levels of gh, ghrelin, gnrhs, ghr, npy, npyrs, pacap, ghrs, igf-I, igf-Ir, and igfbp3 were examined by real- time PCR.The results show that no significant change in plasma GH and IGF-I levels in fish fed with diets containing 30 μg / gand 300 μg / g genistein. mRNA expression of genes along the GH / IGF-I axis remained unaffected, except for igf-Ir, which was stimulated by the 300 μg / g genistein diet. Home in fish fed the 3 000 μg / g genistein diet, the plasma GH and IGF-I levels decreased, and mRNA expression of gh, ghr2, npyr1, igf-I, and igf-Ir were also significantly depressed. In contrast, npy and igfbp3 mRNA expression were enhanced. This study provides convincing evidence for growth impediment by genistein by disturbing the GH / IGF-I axis in Nile tilapia O. niloticus.