The effect of light exposure during collection and culture of hamster embryos on their subsequent development in vitro was examined. When embryos were collected under dark conditions (70 lux) within 10 minutes and then cultured in a HECM-1 medium in 5% CO2 in air, the developmental rates of 1-cell embryos to the 4- and 8-cell stages were 88.6% (93/105) and 66.7% (70/105), respectively. These rates were significantly higher than those under light conditions (1600 lux): 51.9% (56/108) and 34.3% (37/108). Light irradiation during the culture of 1-cell embryos suppressed subsequent development. The degree of suppression correlated inversely with duration of light irradiation, and light irradiation of 30 minutes or more completely blocked development to the 2-cell stage. When 1-cell embryos were irradiated through a yellow filter, cutting the light wavelengths to less than 500 nm, embryonic development was still suppressed. However, the degree of the suppression varied and 45.7% (53/116), 6.0% (7/116), and 0.9% (1/116) of the embryos developed to the 2-, 4-, and 8-cell stages, respectively, under 30 minute light irradiation. Inhibitory effects of light irradiation on the development of 2- and 8-cell embryos were also observed, showing an inverse correlation with duration; the developmental rates of 2-cell embryos to the 8-cell stage under 0, 10, and 30 minutes of irradiation were 85.6% (107/125), 1.6% (2/122), and 0% (0/129), respectively, and those of 8-cell embryos to the blastocyst stage were 79.8% (91/114), 74.8% (86/115), and 0% (0/110), respectively. These findings indicate that early-stage embryos are sensitive to light exposure; however, severe light exposure adversely affects the development of embryos at any stage. Thus, the protection of embryos from light exposure at all stages of embryo manipulation, from collection to culture, is essential.