In summary, foetal and placental metabolisms have certain characteristics which appear to bridge species differences. The consumption of carbohydrate and amino acids by the foetal lamb is adequate to meet the energy, C and N requirements of the foetus both for oxidated metabolism and for growth. Techniques to study foetal or maternal metabolism or both under conditions of a biological steady state, comparatively free of stress, are now readily available in several large mammals and are becoming so for small mammals. This should permit comparative physiologic studies in animals differing widely in diet and in body size. Hopefully, such studies will clarify those characteristics of foetal metabolism which are unique to a species and those which are common to all foetuses.