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Background and Aims Liposomal bupivacaine (LB) is becoming an important drug for postoperative analgesia and has the potential to increase nerve block duration for up to 72 as the encapsulated bupivacaine is slowly released over time. However, LB may not provide adequate immediate postoperative analgesia. Several studies have shown that mixture of liposomal bupivacaine and plain bupivacaine (Mix) may provide immediate and enduring analgesia, but the pharmacokinetic characteristics have not been assessed.Methods To compare the pharmacokinetic profile and tolerability of liposomal bupivacaine and mixture of liposomal bupivacaine and plain bupivacaine, male rats aged 6–8 weeks were selected to receive either LB or Mix by subcutaneous injection and plasma pharmacokinetic profiles were assayed by Liquid Chromatography with tandem mass spectrometry.The present study was approved by the local ethical committee.Results The analysis showed that compare with LB, Mix possessed higher Cmax (2418.33±373.48 ng/ml vs. 281.13±71.54 ng/ml), shorter Tmax (0.17±0.01 h vs. 0.30± 0.21 h), higher AUC0-t (5293.08±307.20 h ng/ml vs. 1414.13±278.33 h ng/ml) and AUC0-∞ (5448.86±311.72 h ng/ml vs. 1469.19±279.80 h ng/ml), but shorter MRT0-∞(9.33±1.15 h vs. 7.17±0.72 h)( table 1, figure 1).Abstract OP42 Figure 1Concentration-time curve of LB and Mix. (a) Concentration-time curve of LB; (b) Concentration-time curve of mixAbstract OP42 Table 1Pharmacokinetic characteristics of LB and Mix in rats (n=3)Conclusions This study is the first to focus on the pharmacokinetic characteristics of LB and Mix in rodent. Our results showed that Mix exhibited shorter time to peak and higher plasma concentrations, but the analgesia duration may reduce as compared to LB. The study was perfoemed in rats, it may differ from human patients, further study was needed to testify the clinical efficacy of mixture of liposomal bupivacaine and plain bupivacaine.