The effect of allopurinol on the kynurenine formation in three manic depressive patients, two in remission and on in depression, has been studied during long-term treatment with L-tryptophan. The patients were loaded with 100 mg L-tryptophan per Kg body weight prior to and after one week on allopurinol, 300 to 500 mg per day. The area under curve (AUC) of plasma total and free tryptophan increased slightly in the patients in remission but not in the depressive. The former showed no change in the AUC of plasma kynurenine whereas an increase was seen in the depressive. The results suggest that allopurinol does not inhibit the human tryptophan pyrrolase definitely during loadings with L-tryptophan.