In view of the fact that cod liver oil has been found to be a specific in the prevention and cure of rickets both in animals and in man, experiments were undertaken to acquire fuller information regarding the scope of its activity. From both a theoretical and a clinical point of view, it seemed of value to ascertain whether, when cod liver oil is fed to pregnant animals, its active principle passes through the placenta and is imparted to the fetus. Furthermore, it seemed worth while to inquire whether the specific factor is secreted into the milk in quantity sufficient to protect the young. From a nutritional standpoint, the period of lactation must be regarded from two different points of view. The one, just referred to, considers solely the antirachitic virtue of the milk; the other takes into consideration supplementary food that the young may receive directly during this period.