An intrauterine growth retardation model was surgically created in pregnant rats by partial occlusion of the uterine artery in midhorn. The surgery was performed on the seventeenth day of pregnancy, and the animals were killed and studied on the twenty-first day of pregnancy. Statistical analyses demonstrated significant decreases in total weights of the fetuses in the upper horn compared with fetal weights of the lower horn. The greatest reduction in fetal weights occurred in the smaller horns with a fewer number of fetuses.