The aims of this study were to characterize sexual dimorphism in the larynx of adult Xenopus laevis and to determine how sex differences arise during postmetamorphic development. The larger male larynx is a result of greater cell numbers in both cartilage and muscle. The dilator laryngis muscle of the male larynx has 6-7 times more muscle fibers than that of the female. At metamorphosis, the larynx is sexually monomorphic and feminine in phenotype. The DNA content of the male larynx doubles during the first 6 months following metamorphosis; there is no net DNA increase in the female larynx during this time. Both sexes experience a marked increase in laryngeal DNA content and mass between 6 months and adulthood. The number of muscle fibers in the male larynx increases at an average rate of 150 fibers a day during the first 10 months of postmetamorphic development. There is no net change in fiber numbers in the female larynx from metamorphosis to adulthood. Administration of the antiandrogen Flutamide to metamorphic frogs prevents the net addition of laryngeal muscle fibers in males. Thus, we propose that addition of postmetamorphic laryngeal muscle fibers in males is dependent upon the presence of circulating androgens. Exogenous testosterone administration results in an increase in laryngeal mass, DNA content, and cellular proliferation in juvenile frogs. Using [3H]thymidine injections to probe ongoing, as well as testosterone-induced, cell proliferation, we conclude that cellular proliferation is regulated differently in males and females during development. Thus androgen-induced proliferation is one cellular mechanism responsible for the sexual dimorphism observed in adults.