Previous work has shown that there is a positive correlation between the induction of Hsp70 and its transient nuclear localization and the acquisition and loss of induced thermotolerance in postimplantation rat embryos. To determine whether Hsp70 is sufficient to induce thermotolerance in postimplantation mammalian embryos, we used a transgenic mouse in which the normally strictly inducible Hsp70 is constitutively expressed in the embryo under the control of a β€-actin promoter. Day 8.0 mouse embryos heterozygous for the Hsp70 transgene were not protected from the embryotoxic effects of hyperthermia (43Β°C); however, homozygous embryos, expressing approximately twice as much Hsp70 as heterozygous embryos, were partially protected (increased embryo viability) from the embryolethal effects of hyperthermia. Although the viability of transgenic embryos was significantly increased compared with that of nontransgenic embryos, this protection did not extend to embryo growth and development. To determine whether the failure to achieve a more robust protection was related to the expression of insufficient Hsp70 in transgenic embryos, we undertook experiments to determine whether the level of Hsp70 correlated with the level of thermotolerance induced by various lengths of a 41Β°C heat shock. A 41Β°C, 5-minute heat shock failed to induce Hsp70 or thermotolerance, a 41Β°C, 15-minute heat shock induced Hsp70 and a significant level of thermotolerance, while a 41Β°C, 60-minute heat shock induced an even higher level of Hsp70 as well as a higher level of thermotolerance. Quantitation of Hsp70 levels indicated that thermotolerance was associated with levels of Hsp70 of 820 pg/Β΅g embryo protein or greater. Subsequent quantitation of the amount of Hsp70 expressed in homozygous transgenic embryos indicated a level of 577 pg/Β΅g embryo protein, that is, a level below that associated with induced thermotolerance. Overall, results presented indicate that Hsp70 does play a direct role in the induction of thermotolerance in postimplantation mouse embryos; however, the level of thermotolerance is dependent on the level of Hsp70 expressed.