AbstractRabbit embryos were subjected to heat‐stress during the first cleavage division. Those embryos to be heat‐stressedin vitrowere recovered at 19 hours after mating and incubated at either 38° or 40°C for six hours, then returned to the original doe. Others were heat‐stressedin vivoby subjecting the doe to a temperature of either 21° or 32°C for 28 hours beginning four hours before mating. Four days after mating embryos were recovered and scored for normality. Those which appeared to be normal were deposited into one uterine horn and the others were deposited into the opposite horn of a synchronous pseudopregnant recipient. Numbers of implantation sites and living fetuses were determined twelve days after mating.There was a significant (P<0.01) reduction in the proportion of ova which cleavedin vitrobut no difference due to incubation temperature. Four days after mating significantly (P<0.01) fewer of the embryos exposed to the higher temperature were classified as normal (49% and 45%vs87% and 74% for ova stressedin vivoorin vitro, respectively). Of the embryos classified normal, 96% and 91% implanted in the 21° and 32°C group, respectively. But 50% of the abnormal embryos also formed implant sites in the 32°C group, however, only 12% of these sites contained a viable fetus. The proportion of embryos forming implantation sites or viable fetuses depended upon the amount of stress that the embryo had been subjected to early in its development. Morphology of a 4‐day old embryo is not indicative of its ability for subse