Fentanyl is a short-acting synthetic opioid with spinal analgesic properties and dose-dependent side-effects. The analgesic effect of opioids is mediated in part through activation of inhibitory descending pain pathways involving nitric oxide (as a central neurotransmitter) through the NO-cGMP system. This NO-cGMP pathway plays an important role in spinal nociception. The aim of the study was to evaluate the effect of transdermal nitroglycerine on the analgesic action of intrathecal fentanyl in patients undergoing abdominal hysterectomy. Patients (n=120) were randomised into one of four groups. All received 3 ml bupivacaine 0.5% plus 0.5 ml of an intrathecal test drug. Twenty minutes after lumbar puncture, a transdermal patch of either nitroglycerine or placebo was applied. Group B received spinal bupivacaine plus saline 0.5 ml and a placebo patch. Group B-N received bupivacaine plus saline 0.5 ml and a nitroglycerine patch. Group F received bupivacaine plus fentanyl 25 microg and a placebo patch. Group F-N received bupivacaine plus fentanyl 25 microg and a nitroglycerine patch. The duration of effective analgesia was longer in group FN (363.53 +/- 34.09 min) compared from the other groups (P < 0.001). The times to two-segment regression in group F-N and group F were 132.87 +/- 31.2 min and 126.40 +/- 26.81 min respectively. The visual analog scale pain score at the time of the first rescue analgesic was similar in all groups. We conclude that nitroglycerine does not result in postoperative analgesia but enhances the analgesic effect of intrathecal fentanyl.