In a controlled clinical study of 20 patients undergoing arthrotomy a single dose of morphine 0.05 mg kg−1 administered extradurally resulted in more pronounced and prolonged pain relief than morphine 0.1 mg kg−1 i.m. in the period immediately after operation. This difference was significant between 2 and 11 h after morphine administration. The maximum analgesic effect for nine patients in the extradural group was obtained about 2 h after injection. Two of 10 in the extradural group experienced urinary retention. Other side-effects were mild for both groups. Plasma concentrations of morphine were measured in five patients in each group. Four hours after administration, morphine was not detectable in plasma in any of the extradural group and in two of the i.m. group. Our study gives further support for the theory that extradural morphine acts on the spinal cord.