Neurotransmission is necessarily difficult to investigate in the brain, particularly in the human brain. However the striking development of anatomical, biochemical and electrophysiological technics now enables a more accurate analysis of the neurones, the neuronal systems and the synapses. The neurones can no longer be considered as unipolarized transmitting nerve units only. They inform other neurones through thousands of nerve terminals, but also receive informations through a great number of afferences the regulation of which involves dendritic release of neurotransmitters. The neurones spread in such spread in such intricated networks that a change in the activity of one neuronal system has an immediate effect on all parts of the brain. We begin to understand the biochemical events taking place within the synapses due to the development of molecular biology, particularly in studies that have characterized the receptor site. The complexity of the organization of the synaptic cleft appears greater inasmuch that one considers the evidence of various types and even sub-groups of defined neurotransmitter receptors (i.e. pre- and postsynaptic, linked or not linked to cyclic-AMP) as well as the discovery of a new class of putative neurotransmitters (the neuropeptides).