Nocicepives messages are vehiculated mostly by the A delta and the C fibres, but not exclusively. Spinal cords cells of layers I and V play a very important role in the transmission of painful messages. Spinothalamic pathways include numerous projections among the reticular formation and the mid-brain. This allows the convergence of miscellaneous informations and creates marrow bonds with the structures implicated in comportmental aspects, such as the hypothalamus and the rhinencephalon. Painful message is under the influence of several controls, permanently. There is a metameric control multisegmental; there is a supremetameric one, the raphe nuclei form the essential stake. The discovery of stereospecific sites for the binding of opiates, led to the discovery of the endogenous opiate peptides. These may act as neuromodulators, specifically at the level of the dorsal-horn. Morphine seems to act like them, re-enforcing the physiological mechanism. Biogenic amines such as serotonin are implicated in the regulation of the painful message.