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Changes in plasma concentrations of sex steroids were examined in male and female zebra finch chicks during the sensitive period for differentiation of sexually dimorphic brain nuclei associated with the control of song. Using a chromatographic separation procedure and radioimmunoassay, androstenedione, testosterone and 5 alpha-dihydrotestosterone were detected in plasma at relatively high concentrations immediately after hatching. There were no sex differences in concentrations of these androgens. An oestrogen, oestradiol-17 beta, which is known to differentiate the song-control system, is raised specifically in the circulating plasma of male zebra finch chicks, and not in females. The surge in oestradiol, which occurs during the first week after hatching, coincides with the period when capacity for differentiation of the song system is maximal. Exposure of the male brain to oestradiol-17 beta could trigger neuronal differentiation.

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