BACKGROUND: The purpose of this study was to evaluate the effect of different doses of fentanyl and morphine on the spread of spinal analgesia produced by bupivacaine. METHODS: 40 patients undergoing arthroscopy or transurethral resection under spinal anesthesia were randomly assigned to receive intravenous 50 or 100 g of fentanyl(F-50, F-100) or 5, 10 mg of morphine(M-5, M-10). 10 min after, we assessed the new levels of analgesia and administered intravenous naloxone 0.4 mg. The levels of sensory analgesia was reassessed 10 min after naloxone. RESULTS: 10 minutes after intravenous opioids, the level of analgesia increased significantly in M-10 group compared with F-50, M-5 group. Naloxone antagonized the effect of opioids on spinal analgesia. CONCLUSIONS: We conclude that systemic opioids enhance the spread of analgesia in a dose dependent manner, and this enhancement was antagonized by naloxone.