Duration of analgesia and side effects following single bolus doses of epidural fentanyl (100 micrograms) or sufentanil (10, 20, 30 or 50 micrograms) were studied in 50 patients who underwent Caesarean section under epidural anaesthesia. Fewer patients experienced pain peroperatively in the fentanyl group than in a joint group of those given sufentanil 20 or 30 micrograms (p less than 0.05). The combined fentanyl and sufentanil 50 micrograms groups had fewer patients in pain than the sufentanil 10 micrograms group at 3 hours after injection (p less than 0.05). Patients given fentanyl also had a longer pain-free interval than those who received sufentanil 10 micrograms (p less than 0.02). The sufentanil 50 micrograms group had more patients asleep than the 10 micrograms group and also had more patients with pruritus than the 10 micrograms or 30 micrograms groups (p less than 0.02). The patients given sufentanil 30 and 50 micrograms had more emetic sequelae than those who received sufentanil 10 and 20 micrograms or fentanyl 100 micrograms (p less than 0.05). There was no detectable excretion of drug into breast milk and no significant respiratory depression at the time of first postoperative analgesia in the patients who received fentanyl or 30 micrograms or less of sufentanil.