Control, low-protein, low-calorie, and low-protein-plus-low-calorie diets were fed to 37, 14, 8, and 16 infant squirrel monkeys, respectively, from ages 2-8 wk. The deficient diets were planned to prevent growth and were designed to examine the short- and long-term effects of defined nutritional restrictions on body weight, food intake, hematocrit, plasma albumin, growth hormone, and cortisol concentrations. During the restriction period, plasma albumin levels were significantly decreased in both groups with a protein restriction component: plasma cortisol levels were significantly increased only in the low protein and low calorie groups. No significant group differences were observed for hematocrit or plasma growth hormone. Results of this study indicate that 1) energy restriction did not seem to modify the protein requirement for weight maintenance and 2) animals fed low-protein diets, even in the face of growth failure, have a mechanism for wasting energy.