We studied the regulation of multiple vitellogenin (Vg) and estrogen receptor (ER) genes, and the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis by estradiol (E2) in liver and gonads of male, female, and E2-treated male tilapia. Plasma Vg and liver expression of three Vg genes and ERα increased in females and E2-treated males, while plasma IGF-I and liver expression of ERß, IGF-I and GH receptor (GHR) were reduced. Expression of Vgs, ERs, IGF-I and GHR was higher in ovary than in testis. These results suggest that E2 acts on the liver to shift energy away from somatic growth towards Vg production.