We developed a newborn mouse behavioral testing method to evaluate the risk of neurotoxicity of environmental toxicants, based on determining a newborn’s motor activity by applying the tare function of an analytical balance. Motor activities of newborn ICR mice exposed prenatally to diethylstilbestrol (DES) at 0.005-0.5 μg/kg/day on days 5 through 18 of gestation were evaluated on postnatal day 1. The activities of male newborns in the 0.05 μg/kg/day group were significantly increased compared to those of the controls, and the increasing tendencies were observed in both sexes of the highest group. The findings indicate that prenatal exposure to low doses of DES causes hyperactivity in newborn mice.