In an open preliminary investigation, pre- and concomitant treatment with a liver pyrrolase inhibitor allopurinol to depressed male outpatients receiving L-tryptophan suggests a rational approach to treatment with this serotonin precursor. The combination is both safe and apparently effective; the rapidity of response in some patients underscores the clinical effects. Implications are that serotonin, or rather an excess in serotonin, is involved in the symptomatic relief of depressions in man.