A partial estrogen receptor- (ER) cDNA had been previously cloned and sequenced in Japanese quail. The 3-and 5-rapid amplification of cDNA ends techniques were used here to identify a cDNA sequence of the quail ER that contains a complete open reading frame. For the first time in an avian species, this cDNA sequence and the corresponding amino acid sequence are described. They are compared with the known ER sequences previously described in mammals and with the ER␣ sequences identified in a selection of mammalian and avian species. The analysis by Northern blotting of the ER mRNA expression in the brain and kidneys revealed the presence of several transcripts. The presence of ER identified by reverse transcriptase-polymerase chain reaction demonstrated a widespread distribution quite different from the distribution of ER␣. The complete neuroanatomical distri-bution of ER mRNA as determined by in situ hybridization with 35 S-and 33 P-labeled oligoprobes is also presented. Transcripts are present in many nuclei implicated in the control of reproduction such as the medial preoptic nucleus, the nucleus striae terminalis, and the nucleus taeniae, the avian homologue of the amygdala. These data demonstrate the presence of ER in a nonmammalian species and indicate that the (neuro)anatomical distribution of this receptor type has been conserved in these two classes of vertebrates. The role of this receptor in the control of reproduction and other physiological processes should now be investigated.