According to the monoaminergic hypothesis of affective and schizophrenic syndromes, a perturbation of influx of precursors of monoamines such as tyrosine (TYR) and tryptophan (TRYP) into the brain might be correlated with the clinical syndromes. The measure of blood cell membrane transports of TYR and TRYP is an interesting peripheral model for the study of these syndromes. This work concerns 395 patients (335 depressed and 60 schizophrenics); 215 of depressed patients were followed after the antidepressant therapy. The transport of TYR and TRYP into erythrocytes was measured as followed: erythrocytes were incubated in plasma for 10 min. at 0 degree and 37 degrees C with radiolabeled TYR and TRYP; facilitated diffusion was calculated by subtracting from the total amount of radioactivity taken up by the cells at 37 degrees C (total uptake) that accumulated at 0 degree C (passive diffusion). The results showed that: 1) The depressive syndromes were characterised by a decrease of facilitated diffusion of TYR, an increase of facilitated diffusion of TRYP and a decrease of the index diffusion of TYR/TRYP. 2) The means of variables were different according to the diagnostic sub-groups (DSM III). In the depressions with alcoholism, the facilitated diffusions were increased compared to depressive syndromes without others diagnosis. The depressions with dementia were in intermediate position. 3) After treatment with antidepressant drugs, the biological variables improved with the clinical improvement, without complete normalization compared to controls. The patients without clinical improvement showed no change in the biological measures. 4) Patients for whom the choice of antidepressant drug was done according to the facilitated diffusions showed a 87% rate of clinical improvement, while this rate was 60% in the other patients. 5) In schizophrenic patients, the biological variables were different than in depressed or normal controls and showed a little decrease of facilitated diffusion of TYR, a marked decrease of diffusion of TRYP and an increased index of diffusion TYR/TRYP.