To study the effects of chronic exposure to testosterone on song behavior development, we administered various amounts of testosterone to juvenile male zebra finches during different periods of song acquisition. We report that testosterone exposure during development profoundly impairs song learning in juvenile males. The effects of the hormone do not seem to be dose related but vary according to the period of life during which testosterone is administered. Exposure to testosterone starting before day 40 and lasting until adulthood decreases the number of syllables in the birds' repertoire and increases the number of phrases per bout of singing. In addition to these changes in the "syntactical" features of song, acoustic abnormalities are also present in the song syllables of males that begin to receive testosterone during the first month of life. Administration of testosterone during only the first 3 weeks of life produces acoustic abnormalities but has no effect on repertoire size or bout structure. Birds receiving the hormone between 20 and 40 d of age develop both acoustic abnormalities and a reduced repertoire size, but their song bouts contain a normal number of song phrases. Exposure to testosterone starting on day 40 produce no behavioral abnormalities. These results indicate that normal song development requires a period of low levels of circulating testosterone and also provide support for the idea that vocal learning entails a progressive sequence of events.