The <i>performances </i>of young rats fed <i>ad libitum </i>during 55 days on casein and gluten diets with 4 and 8 NDpCal% were studied. The gluten diets contained about 1.7 and 3.6 times more protein (g/l00 g diet) than the casein diets. At the low level of utilizable protein the rats on both diets showed a malnutrition syndrome in many ways similar to the protein-calorie malnutrition described in children. At this inadequate protein level gluten was more effective than casein to depress food intake. The rats on gluten showed: poorer growth performance, higher relative weight of kidney and lower relative weight of testes, higher body water and protein contents, higher protein concentration in the liver, lower hepatic glycogen, lower hemoglobin and higher hepatic activity (U/100 g rat) of aspartate aminotransferase than the rats on casein. At the 8 NDpCal% level the rats on gluten showed: higher dietary intake, better growth performance, higher relative weights of kidneys and liver, lower body water and total oxygen consumption, higher hemoglobin and hematocrit, higher activity for aspartate and alanine aminotransferase in the liver (U/100 g rat), than the animals on casein. The rats on both diets with 8 NDpCal% looked lively, but those on gluten appeared to be older than those on casein. In the experimental conditions some of the differences found between the rats on casein and those on gluten: growth and nitrogen gain performance, hemoglobin and hematocrit seem to be related to the intake and not to the quality of the protein consumed; enzyme activities could be in relation to the protein value of the diet, as well as to the concentration of each protein; increased relative weight of liver and kidney could be related to the protein concentration in the diet. Diets of the same protein value with quite different composition may cause adaptive responses in the animal, which must be taken into account if compensating protein quality by quantity is intended in human diets.