It is well established that parenteral treatment of female zebra finch chicks with estradiol masculinizes their song control nuclei and that as adults they are capable of song. Concern over the widespread use of putative environmental estrogens caused us to ask whether oral exposure to estrogens (a natural route of exposure) could produce similar effects. We dosed chicks orally with estradiol benzoate (EB; 1, 10, 100, and 1000 nmol/g of body mass per day, days 5-11 posthatch), the non-ionic surfactant octylphenol (100 and 1000 nmol/g), or the pesticides methoxychlor (100 and 1000 nmol/g) and dicofol (100 nmol/g) and measured their song control nuclei as adults. EB treatment produced increases in song nuclei comparable to that induced by parenteral administration of estrogens. This is the first study of which we are aware to use an oral route of administration, which simulates the natural process of parent birds feeding their nestlings. We conclude that oral exposure to estradiol alters song control nuclei and we report in a related paper (Millam et al., 2001) that such exposure severely disrupts reproductive performance. Although we detected no influence of xenobiotics on induction of song control nuclei the possibility remains that oral exposure to xenoestrogens in high enough doses could affect development.