The blood platelets, which serious arguments permit one to consider as valid models of serotoninergic neurones, were studied in various types of depression and during treatment with antidepressor drugs. The modifications of several platelet parameters were analysed in particular. The level of hydroxyndoles (OH-5-tryptamine or 5-HT for example) is generally lowered. The uptake of 5-HT is also reduced with normalisation by antidepressor treatments; the fixation of mepacrine, sign of the passive uptake of bioamines, is modified in the same direction, especially, in our experience, in maniacs, with a rise during treatment with phenothiazines. Finally, among the platelet enzymes, monoamine oxidase has a reduced activity in bipolar depressed patients, certain maniac depressive psychosis, and endogenic melancholias. Other platelet parameters, (liberation or release, nucleotide levels, 5-HT bonds at membrane level, etc.) may provide in the future, interesting information. In spite of sometimes contradictory data, the platelet model may be of interest, especially from pharmacological point of view.