The expression of TRP53 in blastocysts that had been cultured from the zygote stage in vitro for 90 h was compared with that in blastocysts collected from the uterus in C57BL6 (B6) and in F1 hybrid (B6CBF1) strain mice. In both strains, there was little TRP53 detected in blastocysts collected from the uterus. There was some increased expression in cultured embryos from B6CBF1 mice and marked increased expression in cultured B6 blastocysts. In cultured B6 embryos, there was obvious accumulation of TRP53 within the nuclear region of embryonic cells. Cultured B6 zygotes had significantly poorer rates of blastocyst formation and of capacity to undergo implantation or form viable fetuses than cultured zygotes from B6CBF1 mice or B6 blastocysts collected from the uterus. Trp53^−/− zygotes (B6 background) were significantly more likely to form blastocysts than sibling wild-type embryos, with Trp53 ^/− embryos having an intermediate level of viability (P < 0.01). On transfer of blastocysts to recipient females, Trp53^−/− blastocysts were more likely to form viable fetuses than wild-type or heterozygous sibling blastocysts when the embryos resulted from culture of zygotes (P < 0.001). This shift in viability did not occur when embryos were only subjected to 24 h of culture from the compacted embryo stage. Culture in vitro in the B6 strain caused a marked increase in the expression and nuclear accumulation of TRP53. This expression was a significant cause of the loss of viability that occurs on culture of zygotes from this strain in vitro.