Healthy women who underwent caesarean section under spinal anaesthesia were studied to determine the extent of postoperative analgesia and side-effects produced by low doses of intrathecal morphine. Patients were randomly allocated to receive, in double-blind fashion, 0 mg (group 1: control group), 0.05 mg (group 2), 0.1 mg (group 3), or 0.2 mg (group 4) of morphine, with 10 mg tetracaine in 10% dextrose 2.5 ml. (n = 20 x 4 groups). The effect of intrathecal morphine was examined in terms of the duration until the first supplemental analgesic was needed and the numbers of the doses within the first postoperative 48 h. Pain relief was significantly greater in groups 3 and 4 than in group 1. The incidence of nausea, vomiting and pruritus increased in a dose-dependent manner. No patient developed respiratory depression. Our results suggest that postoperative analgesia lasts more than 24 h with 0.1 mg or 0.2 mg of intrathecal morphine. Since the incidence of side-effects was higher at 0.2 mg, 0.1 mg may be the optimum dose for caesarean section.