Objective To evaluate the effect of morphine on synaptic long-term potentiation (LTP) in the spinal dorsal horn evoked by electric stimulation of sciatic nerve in rats. Methods Twenty-seven healthy male SD rats aged 60-90 d weighing 180-200 g were randomly divided into 4 groups: group Ⅰ control (group C, n=7), group Ⅱ morphine (group M, n=7), group Ⅲ naloxone (group N, n=6), and group Ⅳ morphine + naloxone (group MN, n=7). The animals were anesthetized with intraperitoneal 10% urethane 1 g/kg, intubated and then mechanically ventilated. The bipolar insulated stainless steel recording electrode (impedance 0.5-1 MΩ, diameter 0.1 mm) was inserted into the left side of the spinal dorsal horn at T13-L1 to stimulate the left side of the sciatic nerve. Single square pulses (15 V, 0.5 ms, 1/60 Hz for 30 min) was applied to evoke spinal field potentials. Normal saline 10 μl, morphine 10 μl (15 μg/μl), naloxone 10 μl (2.5 μg/μl), and the mixture 10 μl of naloxone 5 μl (2.5 μg/μl) and morphine 5 μl (15 μg/μl) was gradually instilled over 2 rain in the 4 groups respectively. Five minutes later, high-frequency and intensity tetanic stimulation (30-40 V, 0.5 ms, 100 Hz, given in 4 trains of 1-s duration at 10-s intervals) was used to induce LTP. Then single square stimuli (15 V, 5 ms, 1/60 Hz) were applied to the sciatic nerve for 210 min. The amplitude and latency period of the field potential were recorded 30 min before tetanic stimulation, and 0-30, 35-60, 65-120 and 125-210 min after titanic stimulation. Results Compared with group C, the amplitude of the field potential was significantly decreased and the latency period prolonged in group M and MN, but there was no significant difference in the above indices between group N and C. Compared with group M, the amplitude of the field potential was significantly increased and the latency period shortened in group MN. Compared with those 30 min before the tetanic stimulation, the amplitude of the field potential was significantly increased and latency period shorted at the time points after the tetanie stimulation in group C and N, the amplitude of the field potential was significantly decreased and latency period prolonged at the time points after the tetanie stimulation in group M, and the amplitude of the field potential was significantly increased 0-30 and 35-60 min after the tetanic stimulation and latency period shortened 0-30 min after the tetanie stimulation, the amplitude of the field potential was significantly decreased and latency period prolonged 65-120 and 125-210 min after the tetanic stimulation in group MN. Conclusion Morphine can inhibit synaptic LTP in the spinal dorsal horn evoked by electric stimulation of sciatic nerve in rats, and it may be one of the mechanisms of the central sensitization inhibition.