AbstractWhen female mice of the Jcl:ICR strain were mated with a male either (1) continuously throughout the day or (2) overnight or (3) during 2 hours in the morning at light period, the interlitter variability of fetal body weight on the 18th day of gestation was smallest in the group with short‐period mating. Thus, in the embryonic stage‐specific teratological experiments, this mating schedule is advised. Even for routine reproductive toxicity testing protocols, the short‐period mating may be preferable for the purpose of increasing test sensit