Contents Apoptosis has been shown to be an important regulator of endometrium function. To clarify the regulation of apoptosis in the cat endometrium during the normal oestrus cycle, the expressions of the apoptosis-related proteins (Bcl-2 and Bax) and their correlation to the inhibitor of apoptosis protein Survivin were analysed using immunohistochemistry. The TUNEL technique (TdT-mediated dUTP nick end labelling) was also used to detect DNA fragmentation characteristic of apoptotic cells. The results demonstrated that TUNEL labelling is not effective for the detection of apoptosis in cat endometrium. Survivin was expressed in the luminal and glandular epithelial cells of cat endometrium during all phases of the oestrus cycle. Survivin was localized in both the cytoplasm and nuclei of superficial and deep uterine gland cells during the luteal phase, while only cytoplasmic staining was observed during the follicular and anoestrus phases. Bax immunoreactivity in the cytoplasm of luminal and glandular epithelial cells as well as the smooth muscle cells of blood vessels was weak in the anoestrus phase. Compared with anoestrus, the intensity of Bax immunostaining was moderate in the follicular phase and increased dramatically in the luteal phase. Bcl-2 immunostaining in the cytoplasm of luminal and glandular epithelial cells was moderate in the anoestrus phase. During the early follicular phase, cytoplasmic Bcl-2 immunostaining was detected mostly in glandular epithelial cells. In the mid-follicular phase, in glands, the amount of Bcl-2 protein increased progressively from the superficial to the deep layer. In contrast, the expression of Bcl-2 decreased in the secretory phase, being very low or absent in the mid- and late luteal phases. The overall results suggest that Survivin, Bax and Bcl-2 proteins may cooperatively contribute to cell apoptosis and cell proliferation in the cat uterus during the oestrus cycle.