During blastocyst implantation, cellular degeneration of decidual tissue spreads from the antimesometrial region to the mesometrial region to accommodate the developing conceptus. To identify the mechanism of decidual regression and its tissue localization, decidual formation was induced in either intact pseudopregnant rats or ovariectomized rats treated with steroids. Antimesometrial and mesometrial cells were separated by elutriation and cultured for 48 h before DNA analysis. Nucleosomal DNA fragmentation was observed in antimesometrial cells, whereas DNA fragmentation was less marked in mesometrial cells. To determine whether a similar pattern of apoptosis occurred during decidual development in vivo, decidua tissues from antimesometrial and mesometrial regions were obtained on days 8-14. Nucleosomal DNA fragmentation was first detected on day 10 in the antimesometrial region, with fragmentation in the mesometrial region delayed by at least 24 h. DNA fragmentation increased in both tissues with time, but was always more pronounced in antimesometrial cells. Sulfated glycoprotein-2 (SGP-2) has been associated with apoptosis, and its expression was examined by Northern analysis. Levels of SGP-2 messenger RNA (mRNA) were detected in the antimesometrial region on day 9 and increased markedly by day 10; mesometrial expression was delayed 24 h. Levels of SGP-2 mRNA in both regions decreased before the onset of DNA fragmentation. In contrast, cathepsin-D expression, detected by immunohistochemistry, was localized to few mesometrial and antimesometrial cells between days 9-12, with all cells showing extensive staining by day 14. To determine whether changes in progesterone control mechanisms initiated decidual apoptosis, plasma progesterone levels were measured by RIA, and progesterone receptor mRNA levels in decidual tissues were examined by reverse transcription-polymerase chain reaction. Both parameters remained unchanged during decidual growth and regression. These results provide biochemical evidence that decidual regression occurs by apoptosis initiated in the antimesometrial region and with progression to the mesometrial region. Apoptotic cell death occurs despite high levels of plasma progesterone and high levels of progesterone receptor message in decidual tissue.