The learned singing behavior in songbirds is sex-steroid-dependent and sexually dimorphic. Estrogen plays a major role in masculinizing the song system in these songbirds. The songbird brain synthesizes large amounts of estrogen, which, in the case of zebra finches, have been found to enter the systemic circulation. Aromatase cytochrome P450 is the key enzyme catalyzing the conversion of androgens to estrogens. We have cloned a novel alternatively spliced form of aromatase cDNA expressed predominantly in the zebra finch brain. We have also isolated and characterized the gene coding for zebra finch aromatase which spans 20kb in length. The alternate forms of aromatase mRNA (ARO) differ in their 5'-untranslated regions encoded by either exon 1a or 1b. The putative promoter sequences controlling the regulation of the alternate forms of ARO in zebra finches contain consensus binding sites for various transcription factors. While both the promoters have binding sites for SRY-like transcription factor, a binding site for SF-1 is present only in the promoter 1b active in the ovary. Intriguingly, a 55bp segment within the promoter 1a sequence appears to be highly conserved among zebra finch, mouse and human aromatase promoters active in the brain.