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In the present investigation, the localization of proteins involved in ovarian apoptosis were studied throughout the estrous cycle in the presence of fluctuating hormone levels. Fas, Fas ligand, Bcl-2, Bax and caspase-3 mRNA expression and proteins were detected in all ovarian tissue extracts, though the amount of protein varied with the phase of the estrous cycle. Fas, Bax and caspase-3 protein levels were highest at diestrus and decreased thereafter towards metestrus. In contrast, Fas ligand and Bcl-2 protein levels were lowest at diestrus and increased toward metestrus. Immunohistochemistry revealed that the staining of the anti-apoptotic protein Bcl-2 was more pronounced in healthy preantral follicles than in atretic follicles. In contrast, the pro-apoptotic proteins Fas, Fas ligand, Bax and active caspase-3 were more predominantly present in atretic follicles. In the ovarian surface epithelium (OSE), Fas, procaspase-3 and Bcl-2 immunostaining appeared independent of the phase of the estrous cycle. Fas ligand and Bax staining was detected particularly during proestrus in OSE cells surrounding the ovulatory follicles, while active caspase-3 was observed only in OSE cells at the postovulatory site during estrus. The proportion of luteal cells that stained positively for Fas, Bax and caspase-3 increased with the age of the corpus luteum, while Fas ligand and Bcl-2 immunostaining was strongest in newly formed corpora lutea and decreased thereafter. In conclusion, the components of the Fas signalling pathway were differentially expressed throughout the estrous cycle in a variety of ovarian cell types, which may correspond to hormone dependent survival mechanisms.