Although there have been a vast number of behavioral toxicology studies carried out on adult mice and rats, there have been few neurobehavioral studies utilizing their newborn animals. Thus, we developed a mouse newborn behavioral testing method for evaluating the risk of neurotoxicity of chemicals, by means of determining the newborn's activity using the tare function of an analytical balance. The unstable weighing values resulting from movement of the newborn on the balance recorded by a personal computer every 0.1 s, and the total activities of a newborn from the start time of weighing to individual times of evaluation were calculated. In addition, we confirmed the usefulness of our method by determining the activity of mouse newborns with microcephaly induced by prenatal exposure to a neurotoxicant, methylnitrosourea. Copyright © 2012 John Wiley & Sons, Ltd.