In the last decade, there has been a growing interest in the study of rodent ultrasonic vocalizations (USV) for their usefulness as a measure for behaviorally phenotyping individuals and as a model for the study of neural and molecular mechanisms of vocal communication. The functional role of USV remains poorly understood, however, and most of the research has been heavily influenced by studies in just a couple of species, mainly rats and mice. The study of USV in a less social, non-communal breeder and more aggressive rodent species, such as the golden hamster (Mesocricetus auratus), provides valuable comparative data that shed light on the evolution of this social behavior. The present work studied the vocal repertoire elicited immediately after sexual interactions and how the vocalizations are modulated rapidly by social context and hormonal changes. I found that the vocal repertoire was mainly composed of 1-note simple calls that were highly variable and chaotic. Repertoire composition, entropy and bandwidth of the calls showed modest sex differences. In addition, social environment differentially modulated male vocal behavior in response to a female. Temporal, frequency and amplitude properties of USV changed over time differently when another male had been present during the interaction with a female than when no male had been present. Moreover, social defeat stimulated losers to maintain longer calls and increase frequency parameters over time, but only if the social interaction with the female involved the presence of another familiar neutral male and not in the presence of a familiar winner. Finally, I found rapid modulatory effects of steroids on male vocal behavior in response to a female. In only 30 minutes, a single subcutaneous injection of 17-[beta] estradiol increased temporal parameters of USV, while an injection of dihydrotestosterone increased frequency and amplitude parameters of USV and the mean number of frequency jumps in multi-note calls. Testosterone also had acute effects but those depended on time of call occurrence. Together these findings suggest that USV in hamsters are dynamic signals that are rapidly modulated by social context on a moment-tomoment basis via the nongenomic action of sex hormones. iv