Effects of an amino acid imbalance, caused by adding 3.8% of an amino acid mixture lacking histidine to a diet containing 6% of beef fibrin, on the food intake and food preference of protein-depleted rats are described. After being depleted, animals fed the balanced or imbalanced diets ate equal amounts for 3 days, then both growth rate and food intake of those fed the imbalanced diet dropped. When protein-depleted rats were fed the balanced or imbalanced diet together with a protein-free diet, neither group ate the protein-free diet during the first 3 days. Thereafter, animals fed the imbalanced diet began to eat the protein-free diet in preference to the imbalanced diet, even though the latter would support growth and the former would not. Animals fed ad libitum the balanced diet or the imbalanced diet plus histidine did not eat the protein-free diet at all. These observations indicate that both food intake and food selection are influenced by the amino acid pattern of the diet.