Senescence accelerated mouse-prone (SAMP) mice with a shortened life span show accelerated changes in many of the signs of aging and a shorter reproductive life span than SAM-resistant (SAMR) controls. We previously showed that functional regression (progesterone dissimilation) occurs in abnormally accumulated luteal bodies (aaLBs) of SAMP mice, but structural regression of luteal cells in aaLB is inhibited. A deficiency of luteal cell apoptosis causes the abnormal accumulation of LBs in SAMP ovaries. In the present study, to show the abnormality of Fas ligand (FasL)/Fas-mediated apoptosis signal transducing factors in the aaLBs of the SAMP ovaries, we assessed the changes in the expression of FasL, Fas, caspase-8 and caspase-3 mRNAs by reverse transcription-polymerase chain reaction, and in the expression and localization of FasL, Fas and activated caspase-3 proteins by Western blotting and immunohistochemistry, respectively, during the estrus cycle/luteolysis. These mRNAs and proteins were expressed in normal LBs of both SAMP and SAMR ovaries, but not at all or only in trace amounts in aaLBs of SAMP, indicating that structural regression is inhibited by blockage of the expression of these transducing factors in luteal cells of aaLBs in SAMP mice.