The dependence on protein quality of the protein to calorie ratio in a freely selected diet and the usefulness of giving protein and calories separately in protein evaluation experiments.

I . Concentrated protein diets (60 yo) were prepared with potato protein and wheat gluten. At two different feeding levels and on an ad lib. scale, these diets were given separately from a protein-free diet (supplied ad lib.) to six groups of rats. 2. The relationship between bodyweight and total calorie intake was confirmed and the regression equation was calculated. 3. The rat possesses a regulating mechanism controlling the intake of the non-protein calories needed for any limited or free protein intake. The intake of non-protein calories is completely determined by the nitrogen retention and hence by the quality and quantity of the protein. This holds true for any dietary protein level. On a high-quality protein (potato) and on a lowquality protein (gluten) rats can reach the same optimal weight gain when they are allowed to choose the calorie: protein ratio spontaneously. Under these conditions food efficiency ratios and the utilization of calories for growth are also optimal and almost the same for both groups. T o attain this far more gluten is needed than potato protein, whereas the number of total calories consumed is similar.