The effectiveness of intrathecal morphine (500 micrograms), intrathecal dermorphine (20 micrograms) and routine intramuscular pentazocine (30 mg) in the relief of postoperative pain, was compared in 150 consecutive patients after elective surgery, in a prospective randomized double blind study. The analgesic effect of intrathecal dermorphine was found to be profound and to last significantly longer (43.41 +/- 1.64 hr) than intrathecal morphine (34.45 +/- 2.35 hr) or the routine pentazocine therapy (10.79 +/- 2.23 hr). The mean postoperative hospital stay in the intrathecal dermorphine (5.6 +/- 4) and morphine groups (6.3 +/- 45) was significantly shorter than in the control group (8.7 +/- 42). Side-effects, as urinary retention, vomiting and headache, were not significantly different in the three groups of patients.