The influence of amitriptyline on the plasma level of various neuroleptics was studied in 25 chronic schizophrenic patients. The study lasted 20 weeks. Patients were kept first 4 weeks on their former neuroleptic medication, with amitriptyline added for 12 subsequent weeks, and withdrawn during the last 4 weeks when only the neuroleptic medication was continued unchanged. The plasma level of neuroleptics was assayed by gas-liquid chromatography, once weekly throughout the study. The amitriptyline plasma level was also evaluated once weekly during the 12 weeks of its administration. The mean neuroleptic plasma values for each 4-week period were pooled together in three groups: aliphatic, piperdine and piperazine phenothiazine derivatives. Amitriptyline provoked some increase of the plasma level of all phenothiazine derivatives. This augmentation was significant only transitorily, however. The putative mechanisms of this neuroleptic tricyclic antidepressant interaction are discussed.