Cesium is an alkaline metal close to Rubidium, which is studied in psychopharmacology in the prospect of a possible antidepressive effect. It has stimulating properties of the motor activity on the animal, antagonizes some sedative substances but does not modify either self-stimulation behaviour or induced agressivity. Given in large doses, Cesium presents hypertensive effects bound with a stimulation of the adrenal Epinephrine secretion. As for the synapses Cesium increases the neuro-transmitter release. However the Norepinephrine turn-over is not modified in the central nervous system while that of Serotonin is increased as well as the cerebral concentrations of Tryptophan, Serotonin and 5.HIAA. These very fragmentary data about the toxico-pharmacological properties of Cesium, as well as the very prolonged half-life of this ion (from 50 to 100 days), explain that contrary to Rubidium it has not still been used in man.