Captive groups of house sparrows Passer domesticus formed stable social hierarchies during a two-week test period. Higher-ranking individuals were more aggressive during the first week of testing and had higher plasma levels of luteinizing hormone (LH) and testosterone (T) at the end of that week. Higher-ranking individuals continued to be more aggressive during the second week of testing, but concentrations of LH and T in blood no longer were related to social status. Higher-ranking individuals also did not have higher levels of these hormones on the day the tournament began. Circulating titers of dihydrotestosterone and corticosterone were random with respect to social status. These results support the hypothesis that social rank and aggressiveness are related to circulating levels of T, but this relationship is expressed only when social status is being established or is actively ‘challenged’.