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The authors sought to determine whether developmental differences in the magnitude of embryonic mortality caused by heat stress in vivo are caused by changes in resistance of embryos to elevated temperature. In this regard, responses of oocytes, two‐cell embryos, four‐ to eight‐cell embryos, and compacted morulae to heat shock were compared. An additional goal was to define further the role of cumulus cells and glutathione in thermoprotection of oocytes. In experiment 1, heat shock (41°C for 12 hr) decreased the number of embryos developing to the blastocyst stage for two‐cell (26% vs. 0%) and four‐ to eight‐cell (25% vs. 10%) embryos but did not affect morulae (37% vs. 42%). In experiment 2, exposure of two‐cell embryos to 41°C for 12 hr reduced the number of four‐ to eight‐cell embryos present 24 hr after the end of heat shock (88% vs. 62%). In experiment 3, heat shock reduced the number of two …