The free and total plasma tryptophan concentrations were measured in 41 female depressive. Fifteen patients were subsequently treated with L-tryptophan. The free and total tryptophan were normal in the drug-free depressives and in the depressives on lithium. No significant in free and total plasma tryptophan were found between unipolar and bipolar depressed subjects, or between patients who recovered from depression following L-tryptophan therapy and patients who were resistant. The results suggest that the basal free tryptophan concentration in the plasma of depressed subjects is normal. They furthermore indicate that neither free nor total plasma tryptophan concentrations are valid predictors for the course of a treatment of depressed subjects with L-tryptophan.