In a previous study using an electrical pain model in human patients, naloxone (21 microg/kg) increased the established area of secondary hyperalgesia. Further, administration of naloxone and naltrexone to animals following resolution of an inflammatory condition, have demonstrated a reinstatement of hypersensitivity to a noxious stimulus, indicating latent sensitization. It has thus been speculated that endogenous opioids may play an important role in the transition from acute to chronic pain in humans.