Some years ago the idea occurred to me that we had progressed sufficiently far in our investigations to make it possible to duplicate human experience as relates to diet, with our colony of domestic rats. A systematic attempt has been made to do this. All factors other than diet, such as temperature, ventilation, illumination, cleanliness and opportunity for exercise, were uniform in these experiments. The only factor in the lives of the animals comprising the different groups which was different, was the character of the diet. Such variations as were observed in the physical development, fertility, capacity to produce milk of satisfactory quality and in sufficient amount for the nutrition of the young, length of life and age at which the first signs of senility appeared, can safely be attributed to the character of the diets employed. We had available to aid us in the planning of the diets the extensive data obtained in the application of the biological method for the analysis of a food-stuff. All the more important natural foods employed in human nutrition in the north temperate zone had been examined by this method. In each case we were able to state definitely the nature of the fault or faults in the diet, and the extent to which each departed from approximately the optimum in quality. These experiments afforded data which are of more than ordinary interest when we consider the parallel in physical development, and in such other points of comparison as can be drawn between man and the lower animals.