A technic for determining the nutritional factors that control the secretion of milk in small animals has been devised. It is based upon the assumption that the prolongation of the lactation period may serve as a measure of nutritional factors. In this method litters are reduced to six in number. The nursing mother is allowed alternate periods for feeding and suckling her young. Anemia is prevented from developing in the young by feeding small amounts of iron, copper, and iodine before the animals begin to drink and by placing these elements in the drinking water when the young are older. Under such conditions mothers can rear the young to about the weight of normal weaning and can maintain them until they are about two months old. With litters of five the weight attained is somewhat greater, indicating that six young consume all the milk secreted. There is a considerable individual variation among mothers. Some can extend their lactation period beyond the two months studied. Casein fed at ten and forty per cent levels showed the lower one to be quite inferior. The nursing mothers consumed more food upon the higher protein level when they were allowed an unrestricted intake. In one series the intakes of the higher protein group were made the same as those upon the lower. The higher protein diet still produced superior results. Since much more work must be done before the answer to this protein problem can be given, we are presenting these experiments chiefly as an illustration of the technic.