The corpus luteum (CL) of the pig lacks luteolytic sensitivity (LS) to prostaglandin (PG) F-2α until after day 12 of the oestrous cycle, but the mechanisms underlying this phenomenon are poorly understood. As luteolysis involves apoptosis, we hypothesized that critical apoptotic proteins may be deficient in CLs that lack LS. The specific aim of these studies was to examine mRNA expression and protein levels of apoptosis genes/proteins (BAX/Bax, BCLX/Bcl-x, CASP3/Caspase-3, CASP8/Caspase-8, NFΚB1/NFκB, TP53/p53) in porcine CLs collected at different stages of the oestrous cycle. CLs were collected surgically, mRNA and protein extracted, and expression/levels analyzed by semi-quantitative (SQ) PCR and Western blots, respectively. At the mRNA expression level, only BAX (maximal on day 4) and TP53 (maximal on day 7) showed significant variations during the oestrous cycle. At the protein level, only Bcl-x and Caspase-3 showed significant changes during the cycle; Bcl-x decreased on day 13 and Caspase-3 increased on day 13. It is concluded that apoptosis-associated proteins (i.e. Bcl-x and Caspase 3) may play a critical role in luteolytic sensitivity in the pig.