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Progesterone secretion is required for the growth and differentiation of endometrial stromal cells to form decidual cells. For many cells where a growth factor supports cell growth and proliferation, withdrawal of the growth factor initiates apoptosis. This study determined the time course and tissue location of apoptosis in deciduomal tissue after withdrawal of progesterone or injection of the antiprogestin, RU-486. Total DNA was isolated from decidual tissues at intervals after experimental treatments and separated electrophoretically. Internucleosomal DNA fragmentation characteristic of apoptosis was measured by quantitating levels of the 200 bp fragment. Apoptotic cells in tissue sections were detected by direct immunoperoxidase detection of digoxigenin-labeled DNA. Decidual apoptosis reached maximal levels at 12 h after withdrawal of progesterone or injection of RU-486. Increased concentrations of apoptotic cells were observed at the periphery of the growing deciduoma and in the antimesometrial deciduoma near the luminal epithelium after both treatments. These results suggest the withdrawal of progestin initiates apoptosis in cells at the early stages of decidualization.