Studies were made on tryptophan metabolism in Nagase analbuminemic rats (NAR), a mutant strain established from Sprague-Dawley rats that is characterized by lack of plasma albumin and hyperlipidemia. The total tryptophan concentration in NAR plasma is one fifth of normal, but the plasma free tryptophan concentration and liver and brain tryptophan levels are normal. Radioimmunoelectrophoresis showed that the tryptophan binding protein in NAR plasma is alpha 2-macroglobulin. The effect of oral tryptophan loading on plasma tryptophan and urinary excretion of tryptophan and its metabolites were examined. Differences between the values for NAR and normal rats were not significant except that the disappearance rate of tryptophan from NAR plasma was faster than in normal rats. The effects of several drugs that influence the concentration of non-esterified fatty acid (NEFA) or amino acids in the plasma of normal rats on NAR plasma tryptophan and NEFA concentrations and brain tryptophan metabolism were investigated. These drugs did not alter plasma NEFA and free tryptophan in NAR in the same direction as in normal rats. This difference may be caused by the difference of tryptophan binding proteins in the plasma of these two strains.