Eighty patients undergoing total abdominal hysterectomy under general anaesthesia were randomly divided into four groups to study the dose-response relationship of intrathecal morphine (0, 0.1, 0.3 and 0.5 mg) for postoperative pain relief. Pain scores, as assessed by using the visual analogue scale, revealed that intrathecal morphine provided long-lasting pain relief, was most effective after 0.3 mg and significantly reduced the need for supplementary analgesics (P < 0.05). There was no difference as regards the quality of analgesia or the use of supplementary analgesics between the 0.3 and 0.5 mg groups. Adequate pain relief was not evident after a 0.1 mg dose. There was no incidence of respiratory depression in any of the patients in this study. The incidence of side effects was least following 0.3 mg intrathecal morphine, which we consider to be the optimum dose.