Vocal communication allows animals to express distress, territoriality, and most important, to attract mates. In the African Clawed frog, Xenopus laevis, vocal communication is unique, because not only do males advertise for mates using elaborate click vocalizations, but also females are able to advertise their reproductive readiness by eliciting a "rapping" call. Sex differences in vocal repertoire match sex differences in vocal circuitry. During development, the vocal circuitry in the male grows increasingly sensitive to circulating androgens. Androgens induce tremendous growth in the cartilage and musculature of the peripheral vocal organ, the larynx. Net addition of synapses and motor fibers soon follow providing communication from the motor nucleus in the hindbrain to the vocal organ. The laryngeal motor nucleus, n. IX-X, accumulates androgens that serve to protect n. IX-X neurons from programmed apoptosis. Females, who have low levels of circulating androgens, experience a profound net loss on n. IX-X neurons during this developmental critical period. Once the frogs reach sexual maturity males possess larger and more numerous n. IX-X neurons than females, as well as sizable sex differences in laryngeal robustness and physiology. These measurable sex differences yield vastly different vocal programs. Androgens continue to maintain a critical role in governing breeding season trophic effects and mediating call production. Because male X. laevis are so susceptible to the effects of androgens, they may also be sensitive to the actions of endocrine disrupting chemical agents. The vocal system of X. laevis and its androgen sensitivity thus provide an ideal model for studying changes imposed to the anatomy and physiology of the system by the brominated flame retardant, PBDE-209, a putative anti-androgen and common pollutant. The present studies investigate how PBDE-209 affects the male vocal system when animals are exposed during the androgen-sensitive critical period of vocal system development and during adulthood when the tissues are utilizing androgens to vocalize. PBDE-209 effectively reduces male n. IX-X number and size at higher concentrations after exposure during the organizational critical period. Similar dose-dependent effects were observed in adult n. IX-X neurons. Moreover, PBDE-209 inhibited male-typical vocalization by reducing the number of calls elicited as well as the average call amplitude. These data strongly suggest that PBDE-209 has cytotoxic effects that alter n. IX-X anatomy and function, and may be mediated through pathways that include blocking the androgens necessary for proper vocal system development.