 This study aims to investigate the capability of different analgesia-monitors PhysioDoloris, MetroDoloris, Lille, France, SPI Surgical Plethysmographic Index, GE Healthcare, Helsinki, Finland and AlgiScan, IDMed, Marseille, France in tracing unconscious pain sensations during general anesthesia. Many surgical procedures require general anesthesia. In order to induce hypnosis and analgesia, which are the main components of general anesthesia, the anesthetist usually combines a hypnotic drug with an opioid analgesic. 