Under normal milieu, mammalian oocytes are fertilized, and the zygotes develop into blastocysts in the female reproductive tract. After pick-up of oocytes, exposure to light during in vitro manipulation is unavoidable. Development of mouse zygotes to blastocysts is unaffected by exposure to cool white fluorescent light for 15 min, but the incidence of apoptotic cells in blastocysts is significantly increased by the light exposure. The blastocysts that develop from zygotes exposed to light develop to term fetuses at a lower rate than those developed from zygotes shielded from light. The incidence of apoptoic cells in blastocysts is significantly decreased by addition of SB20358, p38MAPK inhibitor, or Q-VD-Oph, caspase inhibitor. Furthermore, when HB-EGF is added to the culture medium, the total cell number of blastocysts developing from zygotes exposed to light increases, and the incidence of apoptotic cells in blastocysts decreases. These blastocysts develop to term fetuses at the same rate as those developed from zygotes shielded from light. In vitro conditions are stressful, and are not the best environment for eggs. Light is one of the physical factors affecting the function of mammalian embryos and extreme care should be taken concerning its effects.