Data from 103 litters of albino rats were analyzed for the effects of the number of fetuses in the litter and in each uterine horn, resorptions, and intra-uterine position on variations in fetal weight. The right uterine horn tended to contain more fetuses than the left. There appeared to be systemic control over both the number of fetuses in the litter and their distribution between the horns. The number of fetuses per litter and the number per horn had separate and different effects on fetal weight. The number of fetuses in the horn was a more important determinant of fetal weight than was the number in the litter. Fetal weight was higher and the effect of the number of fetuses in the horn was stronger in horns with early resorptions than in horns without resorptions. Resorptions had no effect on the weight of adjacent fetuses or on fetal weight variation in the horn which suggests that resorptions influence fetal growth systemically rather than locally. Fetal mortality was highest in the lower one-third of the uterine horn and lowest in the upper one-third. Fetal weight variation increased as the number of fetuses per horn increased and as mean fetal weight decreased. Intrauterine position was a significant determinant of fetal weight. The heaviest fetuses occupied the middle of the uterine horn while the lightest fetuses were at either end. The importance of these findings in prenatal growth and teratologic studies is discussed.