This study describes the localization of androgen (AR) and progesterone (PR) receptors in the developing ovary in the spiny mouse. The immunohistochemical analysis showed for the first time the expression of AR and PR proteins in the ovary as early as in one day-old females. Both AR and PR were present in germinal epithelium cells, stromal cells as well as in the granulosa and theca layer of ovarian follicles. On days 7, 14, 21, 30, 60 and 90, the distribution of AR and PR depended on the stage of follicular development rather than on the animal's age. A novel observation was that PR protein was detected not only in granulosa cells of preovulatory follicles, but also in the growing and early antral follicles. It was demonstrated that there is a different pattern of AR and PR immunoexpression throughout folliculogenesis. In contrast to AR, whose expression decreased during follicular development, the PR immunostaining increased during this time. It is concluded that androgens and progesterone may play an important role in the early stage of follicular development in the spiny mouse.