Two-chick broods of the blue-footed booby develop a dominance relationship during the junior chick’s first 2 weeks of life, when behaviour of the subordinate chick is conditioned through aggression by its 4-day-older sibling. We used natural and experimental situations to test the Challenge Hypothesis, which predicts a rise in testosterone to regulate aggression only in socially unstable situations, and to investigate the role of corticosterone in the development of submissive behaviour in subordinates. In unmanipulated broods during the first 12 days, we found no differences among singletons, dominants and subordinates in testosterone levels, and corticosterone levels did not differ significantly. In experimental pairings of singleton/dominant and singleton/subordinate, testosterone was not detected and corticosterone increased significantly in dominants and singletons, but not in subordinates. These results indicate that the Challenge Hypothesis does not apply to these young birds. Whether or not circulating corticosterone regulates aggression in chicks awaits further experimentation.