Eight patients undergoing abdominal surgery received epidural analgesia with 0.5% plain bupivacaine at a fixed dose rate (8 ml/h) for postoperative pain relief. Mean sensory level of analgesia (pin prick) was assessed hourly and regressed from a mean preoperative level of T3.7 (+/- 0.3 SEM) to T10 (+/- 0.7) at an average of 8.9 +/- 1.4 h post skin incision (range 4-16 h). Simultaneously, pain scores (4-point scale) increased from zero to 2.1 +/- 0.2. When analgesia regressed greater than or equal to 5 segments, administration of 10 mg morphine intravenously led to a pronounced cephalad increase in sensory analgesia; the initial level of analgesia was achieved in every case despite unchanged bupivacaine infusion. Simultaneously, pain score decreased to 0.3 +/- 0.3. This synergistic effect of systemic morphine on the extent of neural blockade with epidural bupivacaine may have an important role in improving postoperative pain relief.