SummaryThe adverse effects of d-tryptophan and the possibility of it being a surrogate index for predicting adverse effects in rats were investigated. Male rats were fed one of several test diets (20% casein diets with 0% (control), 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% d-tryptophan) for 21 d, and 24-h urine samples on the final day of the experiment were collected. Analyses of food intake and body-weight changes revealed adverse effects to be observed in the group fed the 0.3% d-tryptophan diet. We propose urinary levels of 3-hydroxykynurenine/3-hydroxyanthranilic acid to be surrogate indicators for predicting the adverse effects of d-tryptophan from the break point of body-weight gains and urinary levels of d-tryptophan metabolites. The reaction 3-hydroxykynurenine→3-hydroxyanthranilic acid is catalyzed by the pyridoxal phosphate-dependent enzyme kynureninase. Increasing urinary 3-hydrokykynurenine indicates kynureninase deficiency. Intake of d-tryptophan in rats fed the 0.3% d-tryptophan diet was 0.21 g/kg body weight and feeding of the 0.3% d-tryptophan diet did not elicit adverse effects. Thus, the safe level of d-tryptophan was less than 0.2% in the diet, 0.15 g/kg body weight, in rats.