A considerable amount of evidence indicates that the brain tryptophan level depends on circulating tryptophan. Changes of tryptophan concentration have also been described to modify the activity of benzodiazepines and tricyclic antidepressants. In the present study, a series of psychoactive drugs have been preliminarily screened for their activity on brain and plasma tryptophan levels. The results obtained suggest that tryptophan may be involved to varying extents in the activity of different psychoactive compounds.