Postoperative pain after major orthopaedic operations can be controlled by continuous intrathecal administration of opioids or local anaesthetics. Effective intrathecal analgesia can be achieved through synergism of low doses of the two analgesic drugs and, possibly, less drug-related adverse effects. Therefore, we have evaluated the usefulness of a combined low-dose bupivacaine and morphine infusion in patients undergoing hip and knee arthroplasty. Spinal anaesthesia was induced in 55 ASA I-III patients with 0.5% bupivacaine 2 ml via a 28-gauge spinal catheter (L3-4 interspace) and 0.5-ml increments were given if needed. Intrathecal 24-h infusions consisted of bupivacaine 2 mg h-1 alone (n = 18), bupivacaine 1 mg h-1 alone (n = 18) or bupivacaine 1 mg h-1 combined with morphine 8 micrograms h-1 (n = 19). The interview after 3, 6, 12 and 24 h included assessment of pain at rest and on movement (VAS scale), occurrence of sensory and motor block and nausea/vomiting. Bupivacaine 1 mg h-1 combined with an infusion of morphine provided as good postoperative analgesia as bupivacaine 2 mg h-1, but motor block disappeared earlier (P = 0.01). Patients in the bupivacaine 1-mg h-1 group required more supplementary doses of oxycodone i.m. than the other groups (P = 0.04). Time to first oxycodone dose from the start of intrathecal infusion did not differ between groups. The frequency of nausea and vomiting was similar in all groups. In spite of this, antiemetic medication was required more often in the bupivacaine 1-mg h-1 group (possible because of opioid rescue medication). On the ward, one patient in the bupivacaine 2-mg h-1 group experienced a new increase in sensory block with concomitant hypotension. One patient in the same group had minor decubitus on the heel of the operated leg, probably because of prolonged motor block. We conclude that intrathecal infusion of a combination of bupivacaine 1 mg h-1 and morphine 8 micrograms h-1 produced adequate postoperative analgesia. Unfortunately, postoperative nausea and vomiting was a frequent disturbing adverse effect.