Castration of adult male zebra finches (Poephila guttata, Estrildidae) reduces their singing rate and the tempo of song, but castrates continue to sing song identical in form to preoperative song. Injection or implantation of testosterone propionate (TP) but not of vehicle alone reverses the changes produced by castration. Castration or partial castration also reduces the frequency of courtship, copulation, and aggression. Androgen (TP) replacement reverses these changes, but control injections do not. The persistence of song after castration contrasts with the abolition of song by castration in other birds, and this may be related to the natural history of zebra finches.