AbstractIn rats variation of fetal weights among litters is significantly greater than variation of fetal weights within litters. This phenomenon is called the litter effect. Twenty litters of Wistar rats were studied to determine whether or not variation in the duration of the period allowed for copulation (1.5 hours versus 15 hours) was a cause of the litter effect. Analyses of variance showed that the duration of the mating period did not significantly influence mean fetal weight, intralitter variability, or interlitter variability. It is concluded that the use of a restricted mating period as opposed to the commonly used overnight mating period offers no advantage from the standpoint of fetal weight variability. The need to consider the sample size of both litters and fetuses in teratologic and fetal growth studies is stressed.