When stress was applied on days 3, 5 or 7, embryo quality was not affected. Likewise, studies in sheep show more effect on embryo quality when heat stress is applied on day 0 or day 1 than after day 3. These experiments support earlier research on the period of sensitivity for developing embryos, and extend the thermally sensitive period back to the onset of oestrus, several hours before ovulation. They do not establish how far back in the cycle the maturing oocytes are sensitive. From these studies, it can be concluded that heat stress can adversely affect ovulation rate and fertilization rate. However, the most serious effect is on the quality and viability of embryos if the stress occurs during a thermally sensitive period.