Spontaneous apoptosis was detected in oocytes, morulae, and blastocysts but not in cleavage stage embryos or zygotes, suggesting that apoptosis is dependent on developmental stage. Although p53 transcripts were present throughout early development, nuclear localization of p53 protein was absent suggesting p53-independent apoptosis in blastocysts. Induction of apoptosis in cleavage stage embryos using staurosporine suggests that downstream effectors are present and although they may be inhibited during early development, apoptosis in cleavage stage nuclear transfer embryos also raises the possibility that some upstream effector may be absent in IVF embryos at this stage. Culture with anti-HSP70 reduced development to blastocysts and increased apoptosis, while exposure to increased temperature reduced apoptosis caused by subsequent in vitro culture. Nuclear localization of the p53 protein was detected in blastocysts exposed to increased temperature raising the possibility that the associated decrease in apoptosis may be attributable to p53-mediated cell cycle arrest and/or an increase in heat shock proteins. The ability of bovine embryos to resist stress as development proceeds may be regulated by diverse factors such as the acquisition of apoptotic potential and other protective mechanisms involving heat shock proteins and/or cell cycle arrest.