Based on our previous findings that medroxyprogesterone acetate (MPA) significantly suppressed the formation of preneoplastic mammary hyperplastic alveolar nodules in mice, and that this suppression persisted for some time, we studied the effects of different schedules of MPA treatment on spontaneous mammary tumorigenesis and uterine adenomyosis in SHN virgin mice. Mice received a subcutaneous pellet of MPA every 2 months: I) during the limited period of 1-3 months of age; II) throughout the experiment beginning at 6-8 months of age; and III) throughout the experiment beginning at 2-3 months of age. All treatments significantly enhanced mammary tumorigenesis with little difference in the effects among treatments. The progression of uterine adenomyosis was also stimulated in Experiments I and III, but not in Experiment II. These results are in good accord with our previous observations with progesterone, indicating that MPA has progesterone-like effects on mammary and uterine lesions of mice.