We have compared the efficacy and side effects of extradural morphine with extradural fentanyl for postoperative pain relief. Thirty children (ages 1-16 yr) were allocated randomly to receive, after extradural administration of 0.5% bupivacaine 0.75 ml kg-1 and before surgical incision, extradural morphine 0.75 microgram kg-1 (group M), with an additional dose administered 24 h later or extradural fentanyl 2 micrograms kg-1 (group F) followed by a continuous extradural infusion (during 48 h). There was no major complication (respiratory depression). Pain scores were satisfactory in both groups for 48 h. Ventilatory frequency was greater in group M 20, 21, 22, 23 and 25 h after the beginning of analgesia (P < 0.05). Pruritus, nausea and vomiting were less common with extradural fentanyl (20% vs 53%, P < 0.05 and 0% vs 33%, P < 0.05) than with morphine. Urinary retention occurred with equal frequency (25%) in the two groups. After a bolus of 2 micrograms kg-1, continuous extradural infusion of fentanyl 5 micrograms kg-1 day-1 provided analgesia comparable to that from a daily bolus of extradural morphine 0.75 mg kg-1 and produced fewer side effects.