In order to investigate the effects of testosterone upon the inhibition of ovarian development, larvae of the ranid frog Rana japonica were treated with testosterone at different developmental stages, and their gonads examined at several subsequent larval stages. In larvae of genetic females treated with testosterone during Stages I - III, nests of germ cells (ovarian cords) were less well formed than in normally developing ovaries (Stage V) . Secondary ovarian cavities formed late, and only a few germ cells initiated oogenesis (Stage XV) . In these ovaries, significant proliferation of medullary cells for masculinization took place only after successive rounds of oogenesis had ceased completely. By contrast, in larvae exposed to testosterone during Stages III - X, the primitive nests of germ cells were formed normally, but the new nests corresponding to proliferation of germ cells were poorly formed owing to the presence of only few somatic cells. The replicated germ cells were arranged within primordial nests so as to form a colony (a kind of cyst) ; few cells participated in oogenesis. Although the medullary cells proliferated significantly during the period of treatment with testosterone, their morphogenesis for masculinization took place after the cysts of germ cells had moved completely away from the epithelium. These events suggest that the masculinization caused by treatment with testosterone is induced by stagnation of the development of the somatic cells at gonadal cortex rather than by the proliferation of medullary cells.