The extended theory about a dysfunction of the serotoninergic system in depression and schizophrenia includes the hypothesis of a disturbance in the transport systems of tryptophan and tyrosine from blood to brain. It would be interesting to know if blood cells may be used as a model for the central transport mechanisms of these amino acids. After an oral load, the in vivo distribution of L-tryptophan (50 mg/kg) was studied in the blood plasma, in the different blood cells and its binding to plasma albumin, in six healthy, seven schizophrenic and two depressive subjects. In all the compartments studied, tryptophan reached a peak, 1--2 hours after the load. Before and after the load, the variation of the tryptophan concentration in the erythrocytes was parallel to the plasma free tryptophan, whereas the uptake of this amino acid was higher in leukocytes and thrombocytes than in erythrocytes. However, this model does not show differences between schizophrenic and normal subjects with regard to the transport of tryptophan and tyrosine in these cells.