The primary and secondary sexual characteristics of many species of passerine birds undergo dramatic seasonal variation in response to the change in the length of photoperiod. Among the many physiological processes that undergo seasonal changes, bird song and the song control system underlying it undergo similar seasonal variation in size and function. The mechanisms of this seasonal variation are largely unknown but are at least partially due to steroidal action from the gonads. The present study determined the relative roles played by the gonads and the photoperiodic timing system that controls gonadal development on song control nuclei in the brain of the male house sparrow, Passer domesticus. Sparrows maintained in short photoperiods (SD) possessed small regressed testes. Transfer to long photoperiods (LD) for 6 weeks evoked a dramatic increase in testes size, but, after 20 weeks under the same conditions (LDLD), testes completely collapsed. Song control nuclei HVC and RA were smaller in SD than in LD but regressed only moderately in LDLD. Castration of sparrows in SD reduced the amplitude of the seasonal variation but did not completely abolish it. The data support the view that the song control system of the house sparrow is regulated by the photoperiodic timing system independently of gonadal influence, but that the gonads augment seasonal regulation of song, presumably via steroidal hormone secretion.