A 2-wk feeding study with weanling rats was conducted to investigate the effects of tryptophan supplementation of soy-based infant formulas on protein quality and on the levels of blood and brain tryptophan and brain serotonin and 5-HIAA (5-hydroxyindole-3-acetic acid), a metabolite of serotonin. The powder and liquid concentrate forms of soy-based infant formulas obtained from four manufacturers were tested. The tryptophan contents of the test soy-based formulas (10.0-12.3 mg/g protein) were considerably lower compared to those of human milk (17-19 mg/g protein). The infant formulas were fed as the sole source of protein in diets containing 8% protein, 20% fat, 5% cellulose and adequate amounts of minerals and vitamins. In general, supplementation of the diets containing test formulas with graded levels of L-tryptophan (0.1 or 0.2%) had no effect on protein quality indices based on rat growth such as PER (protein efficiency ratio), NPR (net protein ratio) and relative NPR (RNPR), but resulted in significant (p < 0.05) increases in the concentrations of tryptophan in the plasma and brain, and serotonin and 5-HIAA in brain of rats. The increased concentrations of brain tryptophan and 5-hydroxyindoles (serotonin + 5-HIAA) in rats fed tryptophan-supplemented formulas compared to those fed unsupplemented formulas support the need for further research to investigate the influence of tryptophan supplementation of soy-based infant formulas on tryptophan metabolites and their potential related effects on sleep latency and neurobehavioral developments in infants.