The endometrial layer of the uterus is characterized by continuous cycle of cell growth and apoptosis in response to hormonal changes. Apoptosis is regulated by several apoptotic regulators, but their significance in involuting uterus has not been well understood. For that reason, aim of this study was to investigate possible role of apoptosis-related proteins (bax and survivin) and enzymes (caspase-3 and calpain-1) in the involuting uterus of the rat, using immunohistochemistry. Our results indicated cytoplasmic and nuclear immunostaining for bax, caspase-3, calpain-1 and survivin proteins were found in the endometrial epithelium and stromal cells such as fibroblasts, mast cells and macrophages, and blood vessels; however, calpain-1 immunoreactivity in the endometrial fibroblast was quite weak or absent. Supranuclear punctate bax immunolabelling was also observed in the endometrial fibroblasts and luminal and glandular epithelial cells from days 1st and 3rd following parturition, respectively. Although survivin was localized in the apical cytoplasm underneath the apical membrane of the luminal epithelium on the 1st and 3rd days, it was also localized in the apicolateral membrane and basal cytoplasm on the 10th and 15th days of involution. Immunostainigs demonstrated that expression patterns of all examined proteins varied with structural changes in the luminal epithelium, and number of immunopositive fibroblasts for bax, caspase-3 and survivin increased with advance of postpartum days and reached a maximum on postpartum days 10 and 15. These results suggest that the process of postpartum involution of endometrium may be regulated by apoptotic and non-apoptotic activity of bax, caspase-3, calpain-1, and survivin. (C) 2016 Wiley Periodicals, Inc.