The use of neuraxial opioids has gained popularity over the last few years; they may augment the analgesia produced by the local anesthetic through direct binding with the specific spinal receptors. Morphine, a lipophobic opioid, may not be optimal as an intrathecal drug for intraoperative analgesia because of its slow onset. The lipophilic opioid, fentanyl for instance, if administered intrathecally, its onset is fast and many of its merits by virtue of its lipophilic property may be seen intraoperatively. Seventy five healthy pregnant women who sustained cesarean section under spinal anesthesia were assessed in a randomized fashion. The spinal anesthetic used was 0.5% hyperbaric bupivacaine. Patients were divided into 5 groups, 15 in each group. Fentanyl 0 (Group I), 7.5 (Group II), 10 (Group III), 12.5 (Group IV) and 15 (Group V) micrograms was respectively added to normal saline to make a total volume of 0.3 ml, which was then added to bupivacaine and administered to patients in a randomized fashion. The effect of analgesia, vital signs and side effects were observed every 5 min during operation and every 30 min after operation. It was disclosed that all patients in group V and IV had excellent analgesia intraoperatively, while only 33.3% patients in the control group had an analgesia which was qualified as excellent. Complete analgesia (time from injection to first report of pain) also lasted longer in group IV (201.3 +/- 16.4 min, mean +/- SEM) and group V (210.3 +/- 18.6 min) compared with group I (118.9 +/- 10.4 min). The duration of effective analgesia (time from injection to first parental opioid) was increased with the dose of intrathecal fentanyl above 12.5 micrograms (293 +/- 22.4 min). Both complete analgesia and effective analgesia were not significantly different between group IV and group V. Pruritus was the most common side effect. The incidence of shivering decreased significantly in group IV & V as compared with control group. The combination of bupivacaine with a dose of fentanyl as low as 7.5 micrograms did not produce actual clinical effects. As the dose of fentanyl was increased to 12.5 micrograms or 15 micrograms the quality of surgical analgesia was better and the postoperative analgesia lasted longer. It seemed that the clinical effect might reach its ceiling at the dose of 12.5 micrograms. Pruritus was the most common side effect, but it was mild.