Effects of estradiol-17β (E2) and p-nonylphenol (NP) on mRNA expression of estrogen receptor α (ERα), estrogen receptor β (ERβ) and cytochrome P450 aromatase (P450arom) were examined in the early stage of chicken embryonic gonads. Fertilized eggs were treated with either E2 (1.0mg/egg) or NP (0.01, O.1mg/egg) immediately before incubation (day 0 of incubation) and were incubated at 37.5°C. In the control group the eggs were treated with the vehicle (10% propanediol, PD, 50μl). On day 10 of incubation the gonad was collected and mRNA expression was determined for semi-quantitative assay by the reverse transcription-polymerase chain reaction (RT-PCR). In the control group, ERα and P450arom mRNA showed higher expression in females than in males while ERβ mRNA showed no sexual difference. In the E2 treated group, the expression of ERα and P450arom mRNA markedly increased in males (about 4- and 16-fold, respectively). NP increased mRNA expression of ERα and P450 arom (about 2-fold each) in males. ERβ mRNA expression did not apparently change after the E2 or NP treatment in both sexes. These results suggest that NP induces some female-specific genes in the male gonads during the critical period of sex determination in chicken embryos. © 2002, Japan Poultry Science Association. All rights reserved.