Mitochondria play an important role in the integration and transmission of cell death signals mediated by the Bcl-2 family proteins. Experiments were conducted to determine whether the anti-apoptotic peptides BH4 domain of Bcl-xL (TAT-BH4) and Bax inhibitor peptide (BIP) suppresses heat stress (HS) injury in oocytes by reduction of apoptotic-like events. Cumulus-oocyte complexes (COCs) were matured at 39 degrees C (control) or 41 degrees C (HS) for 21 hr then placed in maturation medium containing 0 or 100 microM BIP in water and 0 or 1 microM TAT-BH4 in dimethyl sulfoxide (DMSO), or a combination of both peptides (BIP + BH4). Peptide effects on embryo development, DNA fragmentation, mitochondrial membrane potential (Delta(Psi)m), and mitochondrial DNA (mtDNA) copy number were measured. All groups were fertilized and cultured in vitro at 39 degrees C for 8 days. Compared to control, HS-treated oocytes induced a decrease in embryo development (P < 0.05), increase in proportion of TUNEL-positive chromatin in oocytes and blastocysts (P < 0.05), and loss of oocyte Delta(Psi)m (P < 0.001). In the presence of BIP or BIP + BH4, development of HS-treated oocytes into blastocysts was increased (P < 0.05). Conversely, COCs matured with TAT-BH4 at 41 degrees C showed reduced embryonic development (P < 0.05). Exposure of HS-treated to each or both peptides resulted in a reduction of TUNEL frequency in oocytes and blastocysts cells derived from these oocytes (P < 0.05). The loss of Delta(Psi)m in HS-treated oocytes was not restored by exposure to BIP + BH4 and there was no effect in mtDNA copy number. In conclusion, the present results show that HS-induced apoptosis in bovine oocytes involves Bax and BH4 domain-dependent pathways.