Estrogen is implicated in sexual differentiation of the avian gonad. Expression of the estrogen receptor and aromatase genes was therefore examined at the time of gonadal sex differentiation in chicken embryos, using reverse transcription and the polymerase chain reaction (RT-PCR). Estrogen receptor (cER) transcripts were detected in the gonads of both presumptive sexes at embryonic days 4.5, 5.5 and 6.5, and in female but not male urogenital tissues at day 3.5. Aromatase (cAROM) transcripts were detected in female but not male gonads from day 6.5 of embryogenesis, and in adult gonads of both sexes. Both female and male embryos thus express cER mRNA before morphological differentiation of the gonads, which begins on day 5, whereas cAROM expression begins at or shortly after the onset of differentiation and is female-specific. Examination of other tissues showed that, in 5.5-day-old embryos, cER expression was limited to the gonads; no transcripts were detected in the mesonephric kidney, liver, brain, hindlimb or heart of either sex. In 9.5-day-old female embryos, cER and cAROM transcripts were present in both the left (ovarian) and the right (regressing) gonads. Altogether, these observations imply that the gonads of both sexes develop the capacity to respond to estrogens early in embryogenesis, before morphological differentiation, whereas the capacity to synthesize estrogens is female-specific and occurs later, at the time of differentiation. These observations are consistent with estrogens having a key role in ovarian development.