Pregnancy rates of 10-15% to a single insemination are common in cattle during the summer in subtropical regions, as compared to about 50% in cool seasons. Research done in the 1960s demonstrated that the embryo was most sensitive during early cell division and gained measurable thermal resistance by the third or fourth cleavage (8—16-cell stage). One might hypothesize that thermal resistance is gained when embryos are able to synthesize specific heat shock proteins such as HSP7O or antioxidants such as glutathione. However, the bovine embryo is able to synthesize HSP7O by the 2-cell stage and has high levels of glutathione (Figure 3). It is possible that these compounds initiate intracellular actions that result in thermal resistance several hours later. The maturing oocyte, the 2-cell embryo and the 4- to 8-cell embryo are sensitive to moderate elevation in temperature (2-3 °C), with the 2-cell embryo most sensitive. The morula does not show sensitivity to this level of stress.