Abstract: Objective: To study the effect of different doses of morphine combined with lornoxicam on CO₂ laser pain threshold in rats and whether morphine and lornoxicam have synergistic effects. Methods: Forty-two male SD rats,weighting 200～250g,were randomly divided into seven groups. Rats in control group received hypodermic injection of 1mL 0.9% NaCl;rats in group M received hypodermic injection of morphine（4mg/kg）;rats in group L received hypodermic injection of lornoxicam（4mg/kg）;rats in groups ML₁,ML₂ and ML₃ received hypodermic injection of morphine combined with lornoxicam（4mg/kg ＆ 4mg/kg,2mg/kg ＆ 2mg/kg,1mg/kg ＆ 1mg/kg）,respectively.Pain behavior of CO₂ laser was assessed at 15,30,60 and 120min before and after drug administration.The percentage of pain threshold was calculated.The interaction type was estimated by Q value provided by Jin Zhengjun’s method. Results: The pain thresholds of rats in all groups except for control group began to increase at about 15min after drug administration and reached their peak at 30min.The effect was maintained till 120min after drug administration in groups M,ML₁ and ML₂.The pain thresholds of rats in groups ML₁,ML₂ and ML₃ showed a statistical difference,and increased with the increasing dose of drugs.The analgesic effect of morphine combined with lornoxicam was significantly higher than that of morphine or lornoxiam alone.The Q value at 60min after drug administration was calculated,and Q＞1.15 indicated that the two drugs had significant synergistic effects. Conclusion: Morphine combined with lornoxicam provides analgesia in a dose-dependent manner.The combination of morphine and lornoxicam has synergistic effects.