Recent Neurobiological (Neurophysiological, Histochemical, Neurochemical and behavioural studies) data have indicated that the analgesic effects of morphine may, at least in part, be explained by two modes of action. A--Morphine has a direct depressive action at a spinal level on the activity of neurones of the grey matter of the dorsal horn which run towards the higher centres of the encephalon. These effects are exerted preferentially on activities induced by the activity of fine non-myelinized fibres (C). These mechanisms are discussed taking into account recent data concerning polypeptides (substance P and encephalins). B--Morphine acts at the level of the brain stem (periaqueductal grey matter, raphian nucleus, etc.) reinforcing the activity of descending bulbo-spinal pathways which block the transmission of painful messages within the cord.