Five groups of male red-winged blackbirds were observed 12 times over 14 days in an aviary setting. An estradiol-treated female was present during the last two observations of each group. Each group of males consisted of a castrated (CA) and an intact (IN) control and six castrated males given one of the following hormone treatments: the aromatizable androgens, testosterone (T) and androstenedione (AE); the nonaromatizable androgens, 5 alpha-dihydrotestosterone (DHT), and androsterone (AN); the estrogen, estradiol (E); or a combination of E + DHT. Castration significantly reduced the frequency of singing and three common vocalizations, chucks, checks, and ips, below the levels shown by IN males. Only hormone treatments which provided both androgenic and estrogenic metabolites (A + E = T, AE, E + DHT) restored normal levels of these behaviors in castrated males. Endocrine status also modulated epaulet display. IN and A + E males were more likely than other males to keep their epaulets constantly exposed. The frequency of sidling and supplanting also varied significantly across hormone-treatment groups, with A + E males showing higher frequencies of these behaviors than other males. T was clearly the most effective hormone treatment in activating hormone-sensitive behaviors in this species. Hormone treatment was more important than size or plumage pattern in determining the outcome of aggressive interactions. In some groups, the dominant male clearly inhibited the performance of hormone-sensitive behaviors by other males. Among A + E males, the frequency of higher intensity song spread displays was highly correlated with the frequency of high-intensity aggressive behaviors and negatively correlated with the tendency to withdraw from other males. Patterns of correlations among behaviors suggest that some calls are more hormone dependent than others, and thus may serve different signal functions.