To investigate the effects of familiarity with opponents on the activation of aggression and dominance by testosterone (T) in white-throated sparrows Zonotrichia albicollis, we studied birds in groups of five or six in outdoor aviaries and in a free-living population. Experiments were conducted between January and April, when plasma levels of endogenous gonadal hormones were low. Previously published experiments (ARCHAWARANON & WILEY, 1988) showed that when subjects were given subcutaneous implants of T, held individually in cages for one week, and then grouped with unfamiliar opponents, they established dominance over controls with empty implants. In contracts, the present study showed that low-ranking birds given subcutaneous implants of T and returned to their original groups and aviaries did not change in dominance rank nor in aggression scores over a period of two weeks. When these birds were regrouped with unfamiliar opponents in new aviaries, their dominance ranks and aggression scores increased. This rise in dominance rankings and aggression scores did not result from any effects of regrouping in the absence of hormonal treatments. The sexes did not differ in their responses to these treatments. In the field, T implants markedly increased some individuals' frequencies of aggression but had little or no effect on dominance relationships. The absence of behavioral changes in stable groupings after hormonal treatment, called social inertia, indicates that birds recognize the relative dominance of previous opponents, at least when encountered in familiar locations. Results from the field experiment thus suggest that white-throated sparrows can recognize at least 20 opponents. Hormonal state has more influence on dominance relationships of strangers than on those of opponents familiar with each other in familiar locations.