A quantitative macroscopic and microscopic investigation was performed upon the placenta of the sheep. Macroscopic data were obtained from 235 cases of single pregnancy, 285 cases of twin pregnancy, 45 cases of triplet pregnancy, and from one case of quadrulet pregnancy. Macroscopic data obtained include: weight of the uterine horns, weight of the fetal membranes, weight of the total cotyledonary mass, and weight of the fetus. Microscopic data were mainly obtained from twin pregnancies (about 30 placentas in each of the nine age groups, group I being excluded from this investigation). The following microscopic data were calculated: vascularization of the caruncles (being the maternal component), degree of branching of the villi, total villous surface, vascularization of the villi, and total villous blood volume.