Ovarian dysgenesis characterized by absence of follicles and corpora lutea associated with hyperplastic interstitial cells can be induced by removal of the thymus at the age of 2–4 days in all strains of mice examined. Thymectomy before this age resulted in death due to wasting disease and thymectomy after the age of 7 days was no longer associated with postnatal ovarian development. Chronologic studies on the ovarian histology of thymectomized mice indicated that degeneration of follicles at early growing stages and subsequent arrest of their development might be fundamental histologic changes evoked by ablation of the thymus. The hypertrophy and hyperplasia of the interstitial cells in the fully advanced dysgenetic ovaries might suggest that these small ovaries perform an endocrinologic function. This assumption is supported by the fact that the uteri of thymectomized mice are not hypoplastic and they undergo rapid atrophic changes after removal of the dysgenetic gonads. Abnormal function of the small ovaries could be supposed from the following: 1) abortive and incomplete luteinization accompanied by marked lymphocyte infiltration in the ovarian tissue after exogenous gonadotropin stimulation, and 2) slight to moderate masculinizing morphology of the submandibular salivary gland of the mice with dysgenetic ovaries. (Endocrinology89: 886, 1971)