The blood platelet may be considered as an interesting model of the serotoninergic neuron in biology and pharmacology. We studied some platelet parameters in neuropsychiatric patients (depressive and schizophrenic ones): serotonin (5-HT) uptake was abnormal for all schizophrenics and lowered for depressive patients; uptake of mepacrine, amarker of 5-HT passive diffusion, was diminished for manic patients; MAO and glutathione peroxidase activities were modified, essentially for paranoid schizophrenics and some alterations of the platelet hydroxyindole level were noticed; during treatment with neuroleptics or antidepressants biological parameters led to ‘normality’. These results are discussed with clinical data and a hypothesis is proposed to join the biological modifications of paranoid schizophren