Morphine sulphate 3 or 4 mg was administered to the extradural space in five volunteers. Morphine 10 or 15 mg i.m. was given as control. Extradural administration resulted in an increase of pain threshold in the legs but not in the forehead. There was no significant depression of carbon dioxide response or deterioration of co-ordination or short-term memory. I.m. morphine caused an increase in forehead pain threshold, depression of the carbon dioxide response and some deterioration of cerebral function. Extradural morphine caused urinary retention in four and inability to ejaculate in three of the subjects. The principal site of action of extradural morphine appears to be regional, at spinal cord level, but it appears that autonomic function as well as pain perception can be affected.